Current research on the genetics of lactic acid production in lactic acid bacteria

Lactic acid derived from lactose is a major by-product of energy production in lactic acid bacteria. The uptake of lactose by these organisms is mediated either by the lactose phosphoenolpyruvate-phosphotransferase system (lactose PEP-PTS), or by a lactose-proton symport system. The disaccharide is then converted to lactate with the concomitant production of ATP. In Lactococcus lactis the genes encoding the lactose PEP-PTS, phospho-beta-galactosidase and the tagatose 6-phosphate pathway enzymes are plasmid encoded, while other genes required for lactate synthesis, including those of the Embden-Meyerhof pathway, are on the chromosome. We have compiled a current list of genes required for lactate synthesis in the lactic acid bacteria that have been cloned and characterized and discuss the present status of genetic research in this area. The analyses of the L. lactic lac operon have yielded one of the most detailed pictures of genetic regulation in the bacterium. The operon has been fully sequenced, the regulatory protein LacR which represses lac operon transcription has been purified and its properties determined, and the operon promoters and operators have been identified. Investigations of chromosomally encoded L. lactis genes have resulted in the identification and characterization of pfk, pyk, idh, tpi, and gap, which encode phosphofructokinase, pyruvate kinase, L-(+)-lactate dehydrogenase, triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase, respectively. All of these enzymes (except triosephosphate isomerase) are known from previous studies to be important in metabolite level regulation of the pathway. pfk, pyk and idh are organized into a tricistronic operon (the las operon), while tpi and gap are in monocistronic units. The las operon is so far unique to L. lactis. A number of investigators have studied the effect of gene dosage on glycolytic flux in lactic acid bacteria and their results are reviewed. We have introduced multiple copies of pfk, pyk, idh and the las operon into L. lactis and report the effect of the increase in gene dosage on enzyme levels and the rate of lactic acid production.