GENE INACTIVATION IN LACTOCOCCUS-LACTIS - BRANCHED-CHAIN AMINO-ACID BIOSYNTHESIS

The Lactococcus lactis subsp. lactis strains isolated from dairy produ cts are auxotrophs for branched-chain amino acids (leucine, isoleucine , and valine), while most strains isolated from nondairy media are pro totrophs. We have cloned and sequenced the leu genes from one auxotrop h, IL1403. The sequence is 99% homologous to that of the prototroph NC DO2118, which was determined previously. Two nonsense mutations and tw o small deletions were found in the auxotroph sequence, which might ex plain the branched-chain amino acid auxotrophy. Nevertheless, the ku g enes from the auxotroph appear to be transcribed and regulated similar ly to those from the prototroph.