Disruption of the gene encoding penicillin-binding protein 2b (pbp2b) causes altered cell morphology and cease in exopolysaccharide production in Streptococcus thermophilus Sfi6.

Genetic and biochemical analysis of exopolysaccharide (EPS) production in lactic acid bacteria has been a growing field of interest in the food industry. We previously identified and characterized a gene cluster composed of 13 genes (epsA to epsM) responsible for EPS production in Streptococcus thermophilus Sfi6. Here we report one further gene, pbp2b, that is connected to EPS production. Mutants with a gene disruption in pbp2b were no longer able to produce EPS, exhibited a reduced growth-rate, and their cell morphology was altered. The predicted gene product showed significant homology to the class B penicillin-binding proteins 2b of Streptococcus pneumoniae, Streptococcus sanguis and Streptococcus mitis involved in peptidoglycan synthesis. Upstream of pbp2b, we further identified two genes which showed significant homology to the E. coli folD and urfl, which is an unidentified open reading frame presumed to be involved in DNA repair. Downstream of pbp2b, we identified a gene that showed homology to the Bacillus subtilis and the Escherichia coli recM or recR which, respectively, are involved in the methyl-dependent DNA mismatch repair. In S. thermophilus, pbp2b and recM were transcribed from their own promoters as monocistronic mRNAs and are therefore organized as independent transcriptional units.