EFFECT OF SIGMA-FACTOR-S (SIGMA-S) ON THE STABILITY OF PENICILLIN-BINDING PROTEIN 3 (PBP3) OF ESCHERICHIA-COLI K12

The stability of penicillin-binding protein 3 (PBP3), a cell septum sy nthesizing protein, was analyzed at different incubation temperatures in three Escherichia coli K12 strains carrying a PBP3-overproducing pl asmid. The stability of PBP3 was significantly reduced in stationary p hase cells shifted to 42 degrees C for 4 h, compared to samples incuba ted at 28 or 37 degrees C. The half-life of PBP3 in the C600 strain wa s 60 min at 42 degrees C, while samples incubated at 28 or 37 degrees C had PBP3 half-lives greater than 4 h. Analysis of the PBP3 content i n mutants deficient in rpoS (coding for the stationary phase sigma fac tor, sigma S) and rpoH (coding for the heat shock sigma factor, sigma 32) genes after shift to 42 degrees C showed that stability of the pro tein was controlled by sigma S but not by sigma 32. These results sugg est that control of the PBP3 levels in E. coli K12 is through a post-t ranscriptional mechanism regulated by the stationary phase regulon. We demonstrated that stability of PBP3 in E. coli K12 involves degradati on of the protein. Moreover, we observed that incubation of cells at 4 2 degrees C significantly reduces the stability of PBP3 in early stati onary phase cells in a process controlled by sigma S.