Cloning and sequence analysis of the gene encoding L-lactate dehydrogenase from Lactococcus lactis: evolutionary relationships between 21 different LDH enzymes.

Lactate dehydrogenase (LDH; EC1.1.1.27) is a key enzyme in the fermentation of milk by lactic acid bacteria used in the dairy industry. An 800-bp DNA fragment containing part of the gene (ldh) encoding LDH was amplified from Lactococcus lactis in a polymerase chain reaction using primers designed from the partial amino acid sequence of a lactococcal LDH. This fragment was radioactively labelled and used to probe a phage lambda library of Lc. lactis genomic DNA. Fragments containing ldh were subcloned from lambda to pUC13 and pUC18 and a 1.2-kb region was sequenced. The deduced aa sequence reveals that the lactococcal LDH is highly homologous to the LDHs of other organisms. The active site and several other domains of unknown function are highly conserved between all LDH enzymes (prokaryotic and eukaryotic). An evolutionary study of LDH sequences clearly divides the prokaryotic from the eukaryotic enzymes except for the Bifidobacterium longum LDH which anomalously groups with the eukaryotic enzymes. The LDHs from Gram-positive bacteria form a separate group from the enzymes from the Gram-negative organisms. The lactococcal LDH is phylogenetically closest to the streptococcal LDH.