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1 MATFORSK, Norwegian Food Research Institute, Osloveien 1, N-1430 Ås, Norway
2 Department of Chemistry, Biotechnology and Food Science, Norwegian University of Life Sciences, PO Box 5003, N-1432 Ås, Norway
3 Microbial Physiology and Genetics, BioCentrum, Technical University of Denmark, Kgs Lyngby, Denmark
Correspondence
Lars Axelsson
lars.axelsson{at}matforsk.no
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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-glucuronidase (GusA) and aminopeptidase N (PepN) expression. Protein production from the synthetic promoters was constitutive, and the most potent promoters gave high protein production with levels comparable to those of native rRNA promoters, and production of PepN protein corresponding to approximately 10–15 % of the total cellular protein. High correlation was obtained between the activities of promoters when tested in L. sakei and L. plantarum, which indicates the potential of the SPL for other Lactobacillus species. The SPL enables fine-tuning of stable gene expression for various applications in L. plantarum.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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). In addition, a variety of Lactobacillus species are members of the normal microbiota of the human gastrointestinal tract and oral cavity, and some also have probiotic effects in humans and animals. Lactobacillus plantarum is among the Lactobacillus species frequently encountered in such niches (Ahrné et al., 1998), and strains with probiotic effects have been reported (Alander et al., 1999; Schultz et al., 2002; Mercenier et al., 2003). L. plantarum is also an important industrial species, due to versatile properties and high flexibility. It is capable of degrading a wide range of carbohydrates, and tolerates relatively low pH. Another species of Lactobacillus important in industrial fermentation is Lactobacillus sakei, which survives a variety of adverse conditions, such as low temperature and pH, high salt concentration, smoke, ethanol, low water activity and radiation (Axelsson & Ahrné, 2000).
The importance of LAB in industrial fermentation and the GRAS (generally regarded as safe) status of most LAB have resulted in an increased interest in their genetics, and a variety of genetic tools have been developed over the last few decades in order to analyse and modify the metabolism of these bacteria. Metabolic engineering of the production of diacetyl and folate by Lactococcus lactis (Hugenholtz et al., 2000; Sybesma et al., 2003), and the production of acetaldehyde by Streptococcus thermophilus (Chaves et al., 2002), are examples. Research involving LAB as cell factories, both for bulk production of chemicals and enzymes, and for in situ production in food, is another upcoming field. The potential of using certain species of LAB, such as L. plantarum, as an in situ producer and deliverer of therapeutic compounds in the human body, has also been investigated (Krüger et al., 2002).
Most advances in the development of gene expression systems for LAB have been for Lc. lactis, and to a lesser extent for lactobacilli, although in recent years several systems have been published that relate to the latter. In particular, gene expression systems based on inducible promoters have been constructed (Pavan et al., 2000; Axelsson et al., 2003; Neu & Henrich, 2003; Sørvig et al., 2003, 2005; Mathiesen et al., 2004). Inducible expression can be preferable in cases where the aim is to overproduce a desired protein at high levels, e.g. at a specific moment during fermentation. However, in other applications, inducible systems are less suitable, i.e. during in situ production in the human body or when steady-state gene expression is required (Jensen et al., 1993), and constitutive promoters could be an alternative for such applications.
Research has been done to identify and isolate constitutive promoters. The most common strategies have been to screen random chromosomal DNA fragments by cloning them in vectors that carry promoterless reporter genes, or to identify strong promoters from housekeeping genes (Pouwels & Leer, 1993; de Vos & Simons, 1994; Mercenier et al., 1994). Jensen & Hammer (1998b) have developed a more efficient approach, in which a library of synthetic promoters for Lc. lactis is obtained by randomization of the spacer sequence that separates the consensus sequences of the promoter. In this library, a wide range of promoter activities is covered in small steps. The approach has been further refined (Solem & Jensen, 2002) and successfully employed for metabolic engineering and metabolic control analysis of Lc. lactis, where fine-tuned expression levels of the studied gene are needed (Koebmann et al., 2002a, b; Solem et al., 2003).
Here, we generalize the Jensen–Hammer approach for obtaining synthetic promoters, and exemplify the method for L. plantarum. A consensus promoter sequence for L. plantarum was derived by aligning its rRNA promoters, and this sequence used as the basis for constructing a synthetic promoter library for L. plantarum. The promoter library obtained provided a wide range of promoter activities with stable protein production, and the most potent promoters yielded protein production at levels comparable to those of native rRNA promoters.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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) was the main host for the synthetic promoter library. The library was also investigated in a plasmid-free variant of L. sakei Lb790, Lb790-X (Schillinger & Lücke, 1989; Møretrø et al., 2005). The high-level, inducible vector pSIP409 (Sørvig et al., 2005), developed for lactobacilli, was used as reference vector and as template for the construction of the library. A derivative of pSIP409, pSIP410, harbouring pepN as reporter gene (Sørvig et al., 2005), was used for constructions of plasmids with selected promoters fused to pepN.
Growth conditions.
E. coli strains were grown at 37 °C in BHI medium (Oxoid) with vigorous shaking. The lactobacilli were grown at 30 °C in MRS medium (Oxoid) without shaking, or in a defined medium for lactobacilli (DML) (Møretrø et al., 1998), with the following modifications: the medium was supplemented with 20 mM glucose and the succinate buffer was replaced with 0·1 M MES buffer. Stability experiments were performed with batch cultures (flasks) in 100 ml DML. Slow stirring with magnets was used to keep the cultures homogeneous. BHI agar and MRS agar (both Oxoid) were used as solid media for E. coli and lactobacilli, respectively. Erythromycin was added to a concentration of 200 µg ml–1 for E. coli and 10 µg ml–1 for lactobacilli.
Plasmid DNA preparation and transformation.
Plasmid DNA from E. coli and lactobacilli was isolated using the QIAprep Miniprep kit (Qiagen). An additional lysis step was introduced for L. plantarum and L. sakei cells, before adding the lysis buffer from the Qiaprep kit: L. plantarum and L. sakei cells were incubated at 37 °C for 10 and 25 min, respectively, in buffer (50 mM glucose, 25 mM Tris/HCl, pH 8·0, 10 mM EDTA) with lysozyme (20 mg ml–1), mutanolysin (40 U ml–1) and RNase (100 µg ml–1). E. coli XL10 Gold cells were transformed according to the manufacturer's procedure, and Lactobacillus strains were electrotransformed according to Aukrust & Blom (1992).
Cloning of synthetic promoter library (SPL) fragments.
The SPL was constructed using standard molecular cloning techniques (Sambrook et al., 1989). Restriction enzymes and T4 DNA ligase were purchased from Promega. PCR was performed with the Expand High Fidelity PCR System Polymerase (Roche Diagnostics) using the standard procedure. The inducible PorfX promoter fragment was excised from pSIP409 using the BglII site in the 5' end of the promoter and the NcoI site included in the gusA start codon. The synthetic promoter fragments were obtained by PCR with pSIP409 plasmid DNA as template. The forward primer SPL1 (DNA Technology A/S, Denmark) (Fig. 2A) carries the BglII site and has homology to the 5' end of the gusA mRNA. The reverse primer gusRT1 (5'-GACTTCGCGCTGATACCAG-3') (MWG AG Biotech) has homology to the interior region of the gusA gene. The resulting PCR product, containing synthetic promoters followed by part of the gusA gene, was digested with BglII and NcoI, and ligated to compatible ends of the digested pSIP409. After transformation into E. coli, all the transformant colonies (approx. 300–400) were pooled and inoculated into 30 ml of medium. After overnight growth, plasmid DNA was isolated from 3 ml of culture and subsequently transformed into L. plantarum. Screening for different promoter activities was achieved by plating the cells onto agar that contained 200 µg ml–1 X-Gluc (5-bromo-4-chloro-3-indolyl--D-glucuronic acid; Biosynth AG), on which the developing colonies ranged in colour from white to dark blue.
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). Cloning of selected rRNA promoters, RNA1-a, rRNA3-a, rRNA4-a, rRNA2-b and rRNA4-b, was performed with the same molecular cloning strategy as that of the SPL, but with specific forward primers for each promoter. The forward primers (MWG) were identical to the SPL1 primer (Fig. 2
) in length, and at the 5'-end restriction site and the 3' end, but contained the interior region sequences from positions –2 to –48 (Fig. 1).
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GusA assay.
The promoter activity was investigated with regard to -glucuronidase (GusA) activity, which was expressed by the gusA reporter gene fused to the promoter. Cultures were grown in defined medium and samples were collected in the exponential phase. The GusA assay was done as described by Axelsson et al. (2003), except for the plasmids that harboured the weakest promoters, for which larger aliquots were concentrated and used in the assay. Activity was calculated as described for -galactosidase by Miller (1972) and expressed as Miller Unit equivalents (MU). The detection limit of the GusA assay has previously been determined to be 6 MU (Sørvig et al., 2003). Mean measurements of the activities from at least four individual cultures of each strain are presented. The stability experiment was performed with two individual cultures of each selected strain.
Construction of PepN strains with selected promoters.
Three promoters from the promoter library (P9, P11 and P27) were excised from their respective pSIP409 vectors by digesting with BglII and NcoI, and ligated to compatible ends of digested pSIP410 harbouring the pepN reporter gene.
PepN assay.
Strains harbouring pSIP410-P9, P11 and P27 were investigated with regard to aminopeptidase activity using L-lysine p-nitroanilide (Sigma) as substrate. The cells were harvested by centrifugation, resuspended in buffer (0·01 M KCl, 0·05 M NaHPO4, pH 7·0, 0·001 M MgSO4) and disrupted by glass beads (106 µm and finer, G-4649, Sigma), essentially as described by van de Guchte et al. (1991). The resulting cell-free extracts were used in the PepN assay. PepN activity was determined according to the protocol described by Exterkate (1984), with the following modifications: reactions were conducted at 30 °C and in 0·1 M Tris/HCl, pH 8·5. Protein concentrations were determined using the RC DC Protein Assay (Bio-Rad) with BSA as standard. The stability experiment was performed with one culture of each selected strain. Each culture was divided in two before harvesting and the corresponding cell-free extracts were prepared and measured for PepN activity on two separate days.
SDS-PAGE analysis.
For protein analysis of PepN, two control strains harbouring pSIP410 were cultivated, and one was induced with 50 ng ml–1 of the SapIP induction peptide (Molecular Biology Unit, University of Newcastle, UK) at OD600 
0·3. All the cells were grown to OD600 1·6 before harvesting. Cell-free protein extracts were obtained as above, and analysed on 12 % SDS-polyacrylamide gels (Bio-Rad). PepN was quantified as a percentage of the total intracellular protein content by scanning the gel with a densitometer (Gel Doc 1000, Bio-Rad).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Alignment of rRNA promoters from L. plantarum and derivation of consensus sequences
The consensus sequences of the synthetic oligonucleotide were based on the alignment of 16S rRNA promoters from the genome-sequenced strain L. plantarum WCFS1 (Fig. 1). Five rRNA genes have been identified in the genome, and we identified two putative promoters (rRNA-a and rRNA-b) upstream of each gene. The rRNA-a promoters were identified nearest to the rRNA genes. TTGACA was the most conserved –35 consensus sequence, and was found in all of the rRNA-a promoters. The –10 sequence was less well conserved in the rRNA promoters, and only three bases (TAXXXT) were conserved. The semi-conserved bases were derived from the rRNA promoters and resulted in a TAWDNT sequence (W=A or T, D=A, T or G, N=A, T, G or C), where N was included to allow for a slightly higher diversity in the –10 sequence. Upstream of the –10 sequence, a conserved TG motif was present at the –15 position of all the rRNA promoters, and the semi-conserved R base (A or G) was included based on the rRNA promoters with a 17 bp spacer sequence. A conserved A base was also identified in the –40 position of the rRNA promoters, and the AGTTG motif was found in almost all of the rRNA-a promoters.
Construction of the SPL
Inducible expression vectors, designated pSIP (Sørvig et al., 2003, 2005) have previously been developed in our laboratory, and show high expression of GusA and PepN in lactobacilli. The pSIP409 vector was selected for the construction of the SPL, because of its low basal activity and high dynamic range of GusA expression. The construction of the SPL involved PCR with the synthetic oligonucleotide SPL1 (Fig. 2A) as the forward primer. A restriction site was included in the 5' end of the oligonucleotide, followed by the derived consensus sequences encompassing the 14 bases of randomized spacer sequence. The 3' end was designed to hybridize to the part of the inducible PorfX promoter that has homology to the 5' end of the gusA mRNA (Fig. 2B). This assured the presence of the same gusA mRNA in both expression systems, and made them directly comparable. The derived PCR fragments, containing a mixture of potential promoter structures, were used to replace the PorfX promoter through cloning into the two restriction sites.
GusA activity of the SPL and comparison with rRNA promoters
Different promoter activities were obtained by screening colonies of L. plantarum NC8 exhibiting different intensities of blue and white when growing on agar plates containing X-Gluc. The colonies were investigated for GusA activity (Fig. 3). A wide range of promoter activities was covered, from below 10 MU to more than 1600 MU, corresponding to a span of 3–4 logs of promoter activity. Comparison with five native rRNA promoters showed that the SPL contained promoters with activities that were in the same range as those of the native rRNA promoters. Interestingly, the activities of the native rRNA promoters were not equally high, but varied from approximately 750 MU to more than 1600 MU. By comparing the most potent promoters from the SPL with maximum induction of PorfX in pSIP409 (Sørvig et al., 2005), it was shown that some of the constitutive promoters had even higher activities than the inducible system.
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). Comparing their DNA sequences with the synthetic oligonucleotide resulted in a number of observations. Twenty-four of the 35 promoters had DNA sequences identical to that of the synthetic oligonucleotide. These included promoters with activities as low as 50 MU and as high as 1600 MU. The remaining promoters had errors in their consensus sequence or a different spacer length than that of the synthetic oligonucleotide, as observed previously (Jensen & Hammer, 1998b). As expected, these errors were most frequent in promoters with the lowest activities. For instance, the entire spacer region and the –35 region were missing in the weakest promoter. Other promoters had deletions or base pair changes in the consensus sequences (–35 and –40) or deletions in the spacers (outside –40, between –35 and –10, or outside –10). Errors in the –15 region were not identified, and only one promoter had an error in the –10 region. However, this was the second weakest of the promoters, and had a base pair change in the –10 sequence, from the conserved A to a T. Two of the promoters (P14 and P45=P46) were not included in Fig. 4, since the inserts were not derived from the oligonucleotide sequence. However, putative –10 sequences were identified, and might explain promoter activities.
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). Stable GusA and PepN expression was measured from all three promoters, perhaps with a slightly lower expression from the potent P11 promoter at high cell densities.
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). This is slightly lower (by approx. 0·2 U ml–1) than the activity we achieved with maximum induction of PorfX in pSIP410. However, SDS-PAGE of cell-free extracts of pSIP410-P11 and maximally induced pSIP410 shows a high amount of PepN protein (Fig. 6). Densitometric analysis indicated that PepN constitutes approximately 10–15 % of the total amount of intracellular proteins of L. plantarum NC8 (pSIP410-P11) and of induced L. plantarum NC8 (pSIP410).
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). A good correlation in the promoter activities of all the promoters was seen between the two species.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The Jensen–Hammer approach, which has successfully been used in Lc. lactis (Jensen & Hammer, 1998b), was used as the basis for generating the SPL. Jensen & Hammer (1998b) extracted extended consensus sequences from a range of sequenced Lc. lactis promoters. In our approach, the consensus sequences were derived by aligning rRNA promoters extracted from the sequenced L. plantarum genome. A comparison of the derived consensus sequences with sequences from the L. plantarum genome resulted in matches with a high number of putative promoters of known genes (data not shown). This indicates that the derived consensus sequence is likely to be active in L. plantarum. The reason for choosing rRNA promoters in finding a consensus sequence is that these are often available or can easily be obtained, even for organisms for which the genomes have not yet been sequenced. In addition, rRNA promoters are known to be among the strongest promoters in most bacteria.
The Jensen–Hammer approach is based on the fact that the spacer sequence between the –35 and the –10 consensus sequences plays an important role in promoter strength. The spacer sequence affects the three-dimensional structure of the promoter, which then affects the efficiency of interactions between the RNA polymerase holoenzyme and the promoter sequence. The spacer sequence was also shown to be important when generating the SPL in L. plantarum. A wide range of promoter activities was obtained, harbouring consensus sequences identical to those of the synthetic oligonucleotide, but with completely different spacer sequences. However, most of the weak promoters had mutations in the consensus sequences or deletions of base pairs in the spacer sequences.
The comparison of the SPL with native rRNA promoters showed that the SPL contains some promoters with activities that are equally high as those of rRNA promoters and others with higher activities. This demonstrates that the range of the SPL also covers promoter strengths that probably are high in nature, since rRNA promoters are known to be among the strongest promoters in bacteria. However, it should be mentioned that the context of the rRNA promoters has been changed compared to their native context on the chromosome, which might affect their strength. The most potent promoters of the SPL also gave higher GusA activities than that measured with maximum induction of pSIP409 (Sørvig et al., 2005). The GusA activities measured are comparable, since both expression systems generate an identical sequence of the gusA mRNA. The amount of PepN protein from the potent P11 promoter was also high (approx. 10–15 % of the total intracellular protein in L. plantarum), but slightly below the amount we found with maximum induction of pSIP410. Higher production of PepN has been obtained previously, reaching 28 and 40 % in L. plantarum (Kahala & Palva, 1999; Mathiesen et al., 2004); however, stable production of PepN has not been reported in these examples.
The stable production of GusA and PepN from selected promoters during exponential growth (Fig. 5) indicates the constitutive potential of the SPL. A weak tendency of decreased GusA and PepN production can be seen from the potent P11 promoter at high cell densities. This has not been investigated, but might perhaps involve changes in plasmid copy number or stability of the reporter gene. However, we have never experimentally observed differences in plasmid copy number. The importance of the reporter gene, e.g. gusA and pepN, has been discussed previously (Sørvig et al., 2005).
A promoter does not necessarily have the same activity in different organisms. The SPL developed for Lc. lactis has been shown to exert a different activity in E. coli (Jensen & Hammer, 1998b). Also, among species of Lactobacillus, promoters have been reported to be species dependent, i.e. Lactobacillus rhamnosus versus Lactobacillus fermentum and L. plantarum (McCracken et al., 2000) and Lactobacillus helveticus versus Lactobacillus casei (Chen & Steele, 2005). Interestingly, when comparing the activities of synthetic promoters in L. plantarum versus L. sakei (Fig. 7), a high correlation in promoter activities was shown. This corresponds to previous experiments with versions of the pSIP expression vector that harbour different inducible promoters, in which there is a correlation between L. plantarum and L. sakei in the production of GusA (Sørvig et al., 2005). Three of the promoters of the SPL were also successfully introduced into another strain of L. plantarum, but fused to another reporter gene, yielding approximately the same ratios of protein expression between the synthetic promoters (data not shown).
There are several advantages to the SPL developed here. The wide range of promoter activities in the SPL allows for the fine-tuning of gene expression, which is important for applications such as metabolic optimization and metabolic control analysis (Jensen & Hammer, 1998a). Another advantage is that the promoters are constitutive, making stability of production easier to achieve than with an inducible system. The stability is also important when performing metabolic control analysis. The fact that some of the synthetic promoters are very strong also makes them well suited for high-level production of proteins. In addition, with constitutive promoters, it is possible to avoid the addition of inducer compounds in large-scale fermentations, and the consequent high cost. The synthetic promoters also have great potential when using live bacteria as antigen delivery vehicles for oral vaccination purposes, especially since the use of L. plantarum and other Lactobacillus species for such purposes has received a lot of attention in recent years (Pouwels et al., 1996, 1998; Havenith et al., 2002). The expression vector pSIP409 was excellent as a basis for constructing the SPL; for example, it has a ‘cassette-like’ structure, which makes it easy to exchange all parts through digestion and ligation. By exchanging the antibiotic resistance marker with a food-grade marker, it can become food grade. It should be mentioned that the synthetic promoters are also regarded as food grade (Jensen & Hammer, 1998b).
In conclusion, we have generalized the Jensen–Hammer approach for obtaining synthetic promoters. The SPL developed in L. plantarum contains promoters with a wide range of promoter activities and with constitutive qualities, which allows for the fine-tuning of steady-state gene expression. The constitutive promoters are also useful for achieving stable protein production, especially when inducible systems are not applicable.
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ACKNOWLEDGEMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Alander, M., De Smet, I., Nollet, L., Verstraete, W., von Wright, A. & Mattila-Sandholm, T. (1999). The effect of probiotic strains on the microbiota of the Simulator of the Human Intestinal Microbial Ecosystem (SHIME). Int J Food Microbiol 46, 71–79.[CrossRef][Medline]
Aukrust, T. & Blom, H. (1992). Transformation of Lactobacillus strains used in meat and vegetable fermentations. Food Res Int 25, 253–261.[CrossRef]
Axelsson, L. (2004). Lactic acid bacteria: classification and physiology. In Lactic Acid Bacteria: Microbiological and Functional Aspects, 3rd edn, pp. 1–66. Edited by S. Salminen, A. von Wright & A. Ouwehand. New York: Marcel Dekker.
Axelsson, L. & Ahrné, S. (2000). Lactic acid bacteria. In Applied Microbial Systematics, pp. 365–386. Edited by F. G. Priest & M. Goodfellow. The Netherlands: Kluwer Academic Publishers.
Axelsson, L., Lindstad, G. & Naterstad, K. (2003). Development of an inducible gene expression system for Lactobacillus sakei. Lett Appl Microbiol 37, 115–120.[CrossRef][Medline]
Chaves, A. C., Fernandez, M., Lerayer, A. L., Mierau, I., Kleerebezem, M. & Hugenholtz, J. (2002). Metabolic engineering of acetaldehyde production by Streptococcus thermophilus. Appl Environ Microbiol 68, 5656–5662.
Chen, Y. S. & Steele, J. L. (2005). Analysis of promoter sequences from Lactobacillus helveticus CNRZ32 and their activity in other lactic acid bacteria. J Appl Microbiol 98, 64–72.[CrossRef][Medline]
de Vos, W. & Simons, G. (1994). Gene cloning and expression systems in lactococci. In Genetics and Biotechnology of Lactic Acid Bacteria, pp. 52–105. Edited by M. Gasson & W. de Vos. Oxford: Chapman and Hall.
Exterkate, F. A. (1984). Location of peptidases outside and inside the membrane of Streptococcus cremoris. Appl Environ Microbiol 47, 177–183.
Havenith, C. E. G., Seegers, J. F. M. L. & Pouwels, P. H. (2002). Gut-associated lactobacilli for oral immunisation. Food Res Int 35, 151–163.
Hugenholtz, J., Kleerebezem, M., Starrenburg, M., Delcour, J., de Vos, W. & Hols, P. (2000). Lactococcus lactis as a cell factory for high-level diacetyl production. Appl Environ Microbiol 66, 4112–4114.
Jensen, P. R. & Hammer, K. (1998a). Artificial promoters for metabolic optimization. Biotechnol Bioeng 58, 191–195.[CrossRef][Medline]
Jensen, P. R. & Hammer, K. (1998b). The sequence of spacers between the consensus sequences modulates the strength of prokaryotic promoters. Appl Environ Microbiol 64, 82–87.
Jensen, P. R., Westerhoff, H. V. & Michelsen, O. (1993). The use of lac-type promoters in control analysis. Eur J Biochem 211, 181–191.[Medline]
Kahala, M. & Palva, A. (1999). The expression signals of the Lactobacillus brevis slpA gene direct efficient heterologous protein production in lactic acid bacteria. Appl Microbiol Biotechnol 51, 71–78.[CrossRef][Medline]
Koebmann, B. J., Andersen, H. W., Solem, C. & Jensen, P. R. (2002a). Experimental determination of control of glycolysis in Lactococcus lactis. Antonie van Leeuwenhoek 82, 237–248.[CrossRef][Medline]
Koebmann, B. J., Solem, C., Pedersen, M. B., Nilsson, D. & Jensen, P. R. (2002b). Expression of genes encoding F1-ATPase results in uncoupling of glycolysis from biomass production in Lactococcus lactis. Appl Environ Microbiol 68, 4274–4282.
Krüger, C., Hu, Y., Pan, Q. & 10 other authors (2002). In situ delivery of passive immunity by lactobacilli producing single-chain antibodies. Nat Biotechnol 20, 702–706.[CrossRef][Medline]
Mathiesen, G., Sørvig, E., Blatny, J., Naterstad, K., Axelsson, L. & Eijsink, V. G. H. (2004). High-level gene expression in Lactobacillus plantarum using a pheromone-regulated bacteriocin promoter. Lett Appl Microbiol 39, 137–143.[CrossRef][Medline]
McCracken, A., Turner, M. S., Giffard, P., Hafner, L. M. & Timms, P. (2000). Analysis of promoter sequences from Lactobacillus and Lactococcus and their activity in several Lactobacillus species. Arch Microbiol 173, 383–389.[CrossRef][Medline]
Mercenier, A., Pouwels, P. & Chassy, B. (1994). Genetic engineering of lactobacilli, leuconostocs and Streptococcus thermophilus. In Genetics and Biotechnology of Lactic Acid Bacteria, pp. 252–293. Edited by M. Gasson & W. de Vos. Oxford: Chapman and Hall.
Mercenier, A., Pavan, S. & Pot, B. (2003). Probiotics as biotherapeutic agents: present knowledge and future prospects. Curr Pharm Des 9, 175–191.[CrossRef][Medline]
Miller, J. M. (1972). Experiments in Molecular Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory.
Møretrø, T., Hagen, B. F. & Axelsson, L. (1998). A new, completely defined medium for meat lactobacilli. J Appl Microbiol 85, 715–722.
Møretrø, T., Naterstad, K., Wang, E., Aasen, I. M., Chaillou, S., Zagorec, M. & Axelsson, L. (2005). Sakacin P non-producing Lactobacillus sakei strains contain homologues of the sakacin P gene cluster. Res Microbiol 156, 949–960.[Medline]
Neu, T. & Henrich, B. (2003). New thermosensitive delivery vector and its use to enable nisin-controlled gene expression in Lactobacillus gasseri. Appl Environ Microbiol 69, 1377–1382.
Pavan, S., Hols, P., Delcour, J., Geoffroy, M. C., Grangette, C., Kleerebezem, M. & Mercenier, A. (2000). Adaptation of the nisin-controlled expression system in Lactobacillus plantarum: a tool to study in vivo biological effects. Appl Environ Microbiol 66, 4427–4432.
Pouwels, P. H. & Leer, R. J. (1993). Genetics of lactobacilli: plasmids and gene expression. Antonie van Leeuwenhoek 64, 85–107.[Medline]
Pouwels, P. H., Leer, R. J. & Boersma, W. J. (1996). The potential of Lactobacillus as a carrier for oral immunization: development and preliminary characterization of vector systems for targeted delivery of antigens. J Biotechnol 44, 183–192.[CrossRef][Medline]
Pouwels, P. H., Leer, R. J., Shaw, M., Heijne den Bak-Glashouwer, M. J., Tielen, F. D., Smit, E., Martinez, B., Jore, J. & Conway, P. L. (1998). Lactic acid bacteria as antigen delivery vehicles for oral immunization purposes. Int J Food Microbiol 41, 155–167.[CrossRef][Medline]
Sambrook, J., Fritsch, E. F. & Maniatis, T. (1989). Molecular Cloning: a Laboratory Manual, 2nd edn. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory.
Schillinger, U. & Lücke, F.-K. (1989). Antibacterial activity of Lactobacillus sake isolated from meat. Appl Environ Microbiol 55, 1901–1906.
Schultz, M., Veltkamp, C., Dieleman, L. A., Grenther, W. B., Wyrick, P. B., Tonkonogy, S. L. & Sartor, R. B. (2002). Lactobacillus plantarum 299V in the treatment and prevention of spontaneous colitis in interleukin-10-deficient mice. Inflamm Bowel Dis 8, 71–80.[CrossRef][Medline]
Solem, C. & Jensen, P. R. (2002). Modulation of gene expression made easy. Appl Environ Microbiol 68, 2397–2403.
Solem, C., Koebmann, B. J. & Jensen, P. R. (2003). Glyceraldehyde-3-phosphate dehydrogenase has no control over glycolytic flux in Lactococcus lactis MG1363. J Bacteriol 185, 1564–1571.
Sybesma, W., Starrenburg, M., Kleerebezem, M., Mierau, I., de Vos, W. M. & Hugenholtz, J. (2003). Increased production of folate by metabolic engineering of Lactococcus lactis. Appl Environ Microbiol 69, 3069–3076.
Sørvig, E., Grönqvist, S., Naterstad, K., Mathiesen, G., Eijsink, V. G. & Axelsson, L. (2003). Construction of vectors for inducible gene expression in Lactobacillus sakei and L. plantarum. FEMS Microbiol Lett 229, 119–126.[CrossRef][Medline]
Sørvig, E., Mathiesen, G., Naterstad, K., Eijsink, V. G. & Axelsson, L. (2005). High-level, inducible gene expression in Lactobacillus sakei and Lactobacillus plantarum using versatile expression vectors. Microbiology 151, 2439–2449.
van de Guchte, M., Kok, J. & Venema, G. (1991). Distance-dependent translational coupling and interference in Lactococcus lactis. Mol Gen Genet 227, 65–71.[CrossRef][Medline]
Received 18 October 2005;
revised 21 December 2005;
accepted 22 December 2005.
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), P11 (
) and P27 (
). Standard deviations are included.
