Sequence analysis and identification of the pyrKDbF operon from Lactococcus lactis including a novel gene, pyrK, involved in pyrimidine biosynthesis.

Three genes encoding enzymes involved in the biosynthesis of pyrimidines have been found to constitute an operon in Lactococcus lactis. Two of the genes are the well-known pyr genes pyrDb and pyrF, encoding dihydroorotate dehydrogenase and orotidine monophosphate decarboxylase, respectively. The third gene encodes a protein which was shown to be necessary for the activity of the pyrDb-encoded dihydroorotate dehydrogenase; we propose to name the gene pyrK. The pyrK-encoded protein is homologous to a number of proteins which are involved in electron transfer. The lactococcal pyrKDbF operon is highly homologous to the corresponding part of the much-larger pyr operon of Bacillus subtilis. orf2, the pyrK homolog in B. subtilis, has also been shown to be necessary for pyrimidine biosynthesis (A. E. Kahler and R. L. Switzer, J. Bacteriol. 178:5013-5016, 1996). Four genes adjacent to the operon, i.e., orfE, orfA, orfC, and gidB, were also sequenced. Three of these were excluded as members of the pyr operon by insertional analysis (orfA) or by their opposite direction of transcription (orfE and gidB). orfC, however, seems to be the distal gene in the pyrKDbF-orfC operon.