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Microbiologie, Vrije Universiteit Brussel, Emile Grysonlaan 1, B-1070 Brussel, Belgium
Vlaams Interuniversitair Instituut voor Biotechnologie, Emile Grysonlaan 1, B-1070 Brussel, Belgium
Research Institute CERIA-COOVI, Emile Grysonlaan 1, B-1070 Brussel, Belgium
ABSTRACT
An arginine biosynthetic gene cluster, argC-argJ, of the extreme thermophilic bacterium Thermus thermophilus HB27 was isolated by heterologous complementation of an Escherichia coli acetylornithinase mutant. The recombinant plasmid (pTHM1) conferred ornithine acetyltransferase activity to the E. coli host, implying that T. thermophilus uses the energetically more economic pathway for the deacetylation of acetylornithine. pTHM1 was, however, unable to complement an E. coli argA mutant and no acetylglutamate synthase activity could be detected in E. coli argA cells containing pTHM1. The T. thermophilus argJ-encoded enzyme is thus monofunctional and is unable to use acetyl-CoA to acetylate glutamate (contrary to the Bacillus stearothermophilus homologue). Alignment of several ornithine acetyltransferase amino acid sequences showed no obvious pattern that could account for this difference; however, the monofunctional enzymes proved to have shorter N-termini. Sequence analysis of the pTHM1 3.2 kb insert revealed the presence of the argC gene (encoding N-acetylglutamate-5-semialdehyde dehydrogenase) upstream of the argJ gene. Alignment of several N-acetylglutamate-5-semialdehyde dehydrogenase amino acid sequences allowed identification of two strongly conserved putative motifs for cofactor binding: a putative FAD-binding site and a motif reminiscent of the NADPH-binding fingerprint. The relationship between the amino acid content of both enzymes and thermostability is discussed and an effect of the GC content bias is indicated. Transcription of both the argC and argJ genes appeared to be vector-dependent. The argJ-encoded enzyme activity was twofold repressed by arginine in the native host and was inhibited by ornithine. Both upstream of the argC gene and downstream of the argJ gene an ORF with unknown function was found, indicating that the organization of the arginine biosynthetic genes in T. thermophilus is new.
Nicolas Glansdorff. Tel: +32 2 526 72 75. Fax: +32 2 526 72 73. e-mail: ceriair@ulb.ac.be
Present address: Department of Microbiology and Immunology, Sherman Fairchild Science Building, 299 Campus Drive, Stanford University, 94305-5124 USA. The EMBL accession number for the sequence reported in this paper is Y10525.
This article has been cited by other articles:
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R. Sanchez, M. Roovers, and N. Glansdorff Organization and Expression of a Thermus thermophilus Arginine Cluster: Presence of Unidentified Open Reading Frames and Absence of a Shine-Dalgarno Sequence J. Bacteriol., October 15, 2000; 182(20): 5911 - 5915. [Abstract] [Full Text]
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