Evidence that the Extracytoplasmic Function Sigma Factor ς(E) Is Required for Normal Cell Wall Structure in Streptomyces coelicolor A3(2)

The sigE gene of Streptomyces coelicolor A3(2) encodes an RNA polymerase sigma factor belonging to the extracytoplasmic function (ECF) subfamily. Constructed sigE deletion and disruption mutants were more sensitive than the parent to muramidases such as hen egg white lysozyme and to the CwlA amidase from Bacillus subtilis. This correlated with an altered muropeptide profile, as determined by reverse-phase high-performance liquid chromatography analysis of lytic digests of purified peptidoglycan. The sigE mutants required high levels of magnesium for normal growth and sporulation, overproducing the antibiotic actinorhodin and forming crenellated colonies in its absence. Together, these data suggest that sigE is required for normal cell wall structure. The role of ς(E) was further investigated by analyzing the expression of hrdD, which is partially sigE dependent. The hrdD gene, which encodes the ς(HrdD) subunit of RNA polymerase, is transcribed from two promoters, hrdDp(1) and hrdDp(2), both similar to promoters recognized by other ECF sigma factors. The activities of hrdDp(1) and hrdDp(2) were reduced 20- and 3-fold, respectively, in sigE mutants, although only hrdDp(1) was recognized by Eς(E) in vitro. Growth on media deficient in magnesium caused the induction of both hrdD promoters in a sigE-dependent manner.