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1Department of Applied Microbiology, Lund Institute of Technology, Lund University, PO Box 124, S-221 00 Lund, Sweden
2Department of Food Technology, Victoria University, PO Box 14428, MCMC Melbourne, Victoria 8001, Australia
ABSTRACT
Maltose metabolism in Lactococcus lactis involves the conversion of β-glucose 1-phosphate to glucose 6-phosphate, a reaction which is reversibly catalysed by a maltose-inducible and glucose-repressible β-phosphoglucomutase (β-PGM). The gene encoding β-PGM (pgmB) was cloned from a genomic library of L. lactis using antibodies. The nucleotide sequence of a 5695 bp fragment was determined and six ORFs, including the pgmB gene, were found. The gene expressed a polypeptide with a calculated molecular mass of 24210 Da, which is in agreement with the molecular mass of the purified β-PGM (25 kDa). A short sequence at the N-terminus was found to be similar to known metal-binding domains. The expression of β-PGM in L. lactis was found to be induced also by trehalose and sucrose, and repressed by lactose in the growth medium. This indicates that β-PGM does not serve solely to degrade maltose, but that it is also involved in the metabolism of other carbohydrates. The specific activity of 
-PGM during fermentation was dependent on the maltose concentration in the medium. The maximum specific activity of β-PGM increased by a factor of 4.6, and the specific growth rate by a factor of 7, when the maltose concentration was raised from 0.8 to 11.0 g I-1. Furthermore, a higher amount of lactate produced relative to formate, acetate and ethanol was observed when the initial maltose concentration in the medium was increased. The specific activity of β-PGM responded similarly to β-PGM, but the magnitude of the response was lower. Preferential sugar utilization and - and β-PGM suppression was observed when L. lactis was grown on the substrate combinations glucose and maltose, or lactose and maltose; maltose was the least-preferred sugar. In contrast, galactose and maltose were utilized concurrently and both PGM activities were high throughout the fermentation.
*Author for correspondence: Peter Rådström. Tel: + 46 46 2223412. Fax: +46 46 2224203. e-mail: Peter.Radstrom@tmb.lth.se
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