CodY-regulated aminotransferases AraT and BcaT play a major role in the growth of Lactococcus lactis in milk by regulating the intracellular pool of amino acids

Aminotransferases, which catalyze the last step of biosynthesis of most amino acids and the first step of their catabolism, may be involved in the growth of Lactococcus lactis in milk. Previously, we isolated two aminotransferases from L. lactis, AraT and BcaT, which are responsible for the transamination of aromatic amino acids, branched-chain amino acids, and methionine. In this study, we demonstrated that double inactivation of AraT and BcaT strongly reduced the growth of L. lactis in milk. Supplementation of milk with amino acids and keto acids that are substrates of both aminotransferases did not improve the growth of the double mutant. On the contrary, supplementation of milk with isoleucine or a dipeptide containing isoleucine almost totally inhibited the growth of the double mutant, while it did not affect or only slightly affected the growth of the wild-type strain. These results suggest that AraT and BcaT play a major role in the growth of L. lactis in milk by degrading the intracellular excess isoleucine, which is responsible for the growth inhibition. The growth inhibition by isoleucine is likely to be due to CodY repression of the proteolytic system, which is necessary for maximal growth of L. lactis in milk, since the growth of the CodY mutant was not affected by addition of isoleucine to milk. Moreover, we demonstrated that AraT and BcaT are part of the CodY regulon and therefore are regulated by nutritional factors, such as the carbohydrate and nitrogen sources.