Effects of the mutant sigma allele rpoD800 on the synthesis of specific macromolecular components of the Escherichia coli K12 cell

Escherichia coli K12 strains containing the mutant sigma allele rpoD800 are temperature-sensitive for growth. We have compared gene expression in isogenic rpoD + and rpoD800 cells during steady-state growth and after temperature shift, in order to define the role of the sigma subunit in vivo. We have shown that sigma synthesis is regulated. After temperature shift-up, sigma behaves like other heat-shock proteins. The stimulation of sigma synthesis by heat shock is greater in mutant than in wild-type cells, possibly because the cell is responding to sigma limitation at high temperature by over-producing sigma. Mutant cells continue protein synthesis for a short time after shift-up and then shut off the synthesis of all proteins in response to the decreasing intracellular concentration of sigma. During the initial period of high protein synthesis, the relative expression of many proteins is changed in mutant cells. We argue that these changes are predominantly an indirect, rather than a direct effect of mutant sigma and are due to a change in the physiological state of mutant cells. Finally, we have shown that degradation of mutant sigma results in a decrease in synthesis of all major messenger RNA and stable RNA species. If other sigma factors are present in exponentially growing cells, they do not appear to be involved in a significant fraction of RNA synthesis.