PRODUCT FORMATION AND PHOSPHOGLUCOMUTASE ACTIVITIES IN LACTOCOCCUS-LACTIS - CLONING AND CHARACTERIZATION OF A NOVEL PHOSPHOGLUCOMUTASE GENE

Maltose metabolism in Lactococcus lactis involves the conversion of be ta-glucose 1-phosphate to glucose 6-phosphate, a reaction which is rev ersibly catalysed by a maltose-inducible and glucose-repressible beta- phosphoglucomutase (beta-PGM). The gene encoding beta-PGM (pgmB) was c loned from a genomic library of L. lactis using antibodies. The nucleo tide sequence of a 5695 bp fragment was determined and six ORFs, inclu ding the pgmB gene, were found, The gene Technology, Victoria expresse d a polypeptide with a calculated molecular mass of 24210 Da, which is in agreement with the molecular mass of the purified beta-PGM (25 kDa ). A short sequence at the N-terminus was found to be similar to known metal-binding domains. The expression of beta-PGM in L. lactis was fo und to be induced also by trehalose and sucrose, and repressed by lact ose in the growth medium. This indicates that beta-PGM does not serve solely to degrade maltose, but that it is also involved in the metabol ism of other carbohydrates. The specific activity of beta-PGM during f ermentation was dependent on the maltose concentration in the medium. The maximum specific activity of beta-PGM increased by a factor of 4.6 , and the specific growth rate by a factor of 7, when the maltose conc entration was raised from 0.8 to 11.0 g l(-1). Furthermore, a higher a mount of lactate produced relative to formate, acetate and ethanol was observed when the initial maltose concentration in the medium was inc reased. The specific activity of alpha-PGM responded similarly to beta -PGM, but the magnitude of the response was lower. Preferential sugar utilization and alpha- and beta-PGM suppression was observed when L. l actis was grown on the substrate combinations glucose and maltose, or lactose and maltose; maltose was the least-preferred sugar, In contras t, galactose and maltose were utilized concurrently and both PGM activ ities were high throughout the fermentation.