2 DIFFERENT DIHYDROOROTATE DEHYDROGENASES IN LACTOCOCCUS-LACTIS

The pyrimidine de novo biosynthesis pathway has been characterized for a number of organisms. The general pathway consists of six enzymatic steps. In the characterization of the pyrimidine pathway of Lactococcu s lactis, two different pyrD genes encoding dihydroorotate dehydrogena se were isolated. The nucleotide sequences of the two genes, pyrDa and pyrDb, have been determined. One of the deduced amino acid sequences has a high degree of homology to the Saccharomyces cerevisiae dihydroo rotate dehydrogenase, and the other resembles the dihydroorotate dehyd rogenase from Bacillus subtilis. It is possible to distinguish between the two enzymes in crude extracts by using different electron accepto rs. We constructed mutants containing a mutated form of either one or the other or both of the pyrD genes. Only the double mutant is pyrimid ine auxotrophic.