Cloning, sequencing and comparison of three lactococcal L-lactate dehydrogenase genes

The conversion of pyruvate to lactate is a key feature of lactococcal strains. The enzyme which facilitates this conversion, L-lactate dehydrogenase (LDH), and the gene which encodes it (Idh), are therefore of great significance. This paper presents the cloning and DNA sequence analysis of three further lactococcal genes which are of key importance in the genetic manipulation of commercial starter strains. The Idh gene from Lactococcus lactis subsp. Lactis biovar diacetylactis BU2-60 has been isolated from a lambda library and sequenced. The Idh gene from L. lactis subsp. cremoris NCDO 762 and that from L. lactis subsp. Lactis IL1403 have been amplified by the polymerase chain reaction (PCR) and sequenced. These DNA sequences and deduced amino acid sequences have been compared with those from L. lactis subsp. Lactis MG1363. The LDHs from L. lactis subsp. Lactis MG1363 and L. lactis subsp. cremoris NCDO 762 are 99.4% homologous. The LDHs from L. lactis subsp. lactis MG1363 and L. lactis subsp. Lactis IL1403 are 96.4% homologous. The LDHs from L. lactis subsp. lactis IL1403 and L. lactis subsp. lactis biovar diacetylactis BU2-60 are 99.9% homologous. Our results provide further evidence that L. lactis subsp. Lactis MG1363 and other L. lactis subsp. Lactis NCDO 712 derived strains should be reclassified as Lactococcus lactis subsp. cremoris.