CLONING, SEQUENCING, AND CHARACTERIZATION OF MULTICOPY SUPPRESSORS OFA MUKB MUTATION IN ESCHERICHIA-COLI

The mukB gene codes for a 177 kDa protein, which might be a candidate for a force-generating enzyme in chromosome positioning in Escherichia coil. The mukB106 mutant produces normal-sized, anucleate cells and s hows a temperature-sensitive colony formation. To identify proteins in teracting with the MukB protein, we isolated three multicopy suppresso rs (msmA, msmB, and msmC) to the temperature-sensitive colony formatio n of the mukB106 mutation. The msmA gene, which could not suppress the production of anucleate cells, was found to be identical to the dksA gene. The msmB and msmC genes suppressed the production of anucleate c ells as well as the temperature-sensitive colony formation. However, n one of them could suppress both phenotypes in a mukB null mutation. DN A sequencing revealed that the msmS gene was identical to the cspC gen e and that the msmC gene had not been described before. A homology sea rch revealed that the amino acid sequences of both MsmB and MsmC posse ssed high similarity to proteins containing the cold-shock domain, suc h as CspA of E. coil and the Y-box binding proteins of eukaryotes; thi s suggests that MsmB and MsmC might be DNA-binding proteins that recog nize the CCAAT sequence. Hence, the msmB and msmC genes were renamed c spC and cspE, respectively. Possible mechanisms for suppression of the mukB106 mutation are discussed.