The pyrH gene of Lactococcus lactis subsp. cremoris encoding UMP kinase is transcribed as part of an operon including the frr1 gene encoding ribosomal recycling factor.

The pyrH gene of Lactococcus lactis subsp. cremoris MG1363, encoding UMP kinase, has been sequenced and cloned. It encodes a polypeptide of 239 amino acid residues (deduced molecular weight of 25951), which was shown to complement a temperature sensitive pyrH mutation in Escherichia coli, thus establishing the ability of the encoded protein to synthesize UDP. The pyrH gene in L. lactis is flanked downstream by frr1 encoding ribosomal recycling factor 1 and upstream by an open reading frame, orfA, of unknown function. The three genes were shown to constitute an operon transcribed in the direction orfA-pyrH-frr1 from a promoter immediately in front of orfA. This operon belongs to an evolutionary highly conserved gene cluster, since the organization of pyrH on the chromosomal level in L. lactis shows a high resemblance to that found in Bacillus subtilis as well as in Escherichia coli and several other prokaryotes