THE CMK GENE ENCODING CYTIDINE MONOPHOSPHATE KINASE IS LOCATED IN THERPSA OPERON AND IS REQUIRED FOR NORMAL REPLICATION RATE IN ESCHERICHIA-COLI

A gene encoding a polypeptide of 25 kDa is located immediately upstrea m of the gene for ribosomal protein S1, rpsA. In high gene copy number , this gene, mssA, was previously found to suppress defects in smbA, w hich is now known to be identical to pyrH, encoding UMP kinase. We sho w here that the 25-kDa polypeptide comprises CMP kinase and propose th at the gene be designated cmk. In a strain deleted for cmk, the pools of CMP and dCMP were elevated approximately 30-fold, We constructed a plasmid from which synthesis of CMP kinase was regulated by the lac pr omoter-operator and measured the synthesis rates for RNA and DNA after induction in the Delta cmk/lacPO-cmk(+) strain. A specific increase i n the rate of DNA synthesis was observed. Further analyses showed that the replication elongation rate was halved in the Delta cmk strain, m ost likely caused by the reductions of the dCTP and dTTP pools to 30 a nd 70%, respectively, of the levels in the parental strain, but that t his was compensated for by a doubling in the frequency of initiation. The Delta cmk strain is viable at 37 degrees C but cold sensitive, The cold sensitivity may be related to defects in the synthesis of phosph olipids or lipopolysaccharides. In addition to the physiological studi es, the region upstream of cmk was sequenced, and 120 codons with stro ng homology to an uncharacterized protein of the speB operon were iden tified.