Abstract

The E. coli ssd mutant is so "sick" that it accumulates suppressor mutations that only partially restore the parental strain phenotype. Based on map location and phenotypic similarities, the ssd was annotated as a cpxA mutation despite unique phenotypes of its own. Here, the ssd mutation and one suppressor mutation are shown located within the cpx regulon, and each strain carrying a cpxR , cpxP or cpxA deletion demonstrates part of the ssd phenotype. The cpxA * gain-of-function and cpxA deletion mutants are shown to have similar phenotypes likely because they both lack the phosphatase responsible for inactivating CpxR. Many deletion mutants are screened from the glycolytic, serine metabolism and Cpx pathways to reveal previously unknown gene products regulating growth on serine. A mucoid phenotype can arise in ssd , cpxR and cpxP mutants, whereas the cpxA mutants alone can have an effect on L-serine deaminase activity---indicating that the Cpx proteins have roles outside of the Cpx two-component regulatory system. RNA expression analysis reveals that the cpxP transcript is increased in ssd mutants, constitutive in the parent strain, and differentially regulated in cpxR and cpxA deletion mutants. Fluorescence microscopy techniques show that ssd mutant filaments do not have a damaged membrane despite a lack of proper DNA segregation, and merodiploid parental and ssd strains having an extra copy of ZipA have improper DNA segregation in cells that make filaments and lyse extensively.