The sigma(s) level in starving Escherichia coli cells increases solely as a result of its increased stability, despite decreased synthesis

The sigma(S) level in starving (stationary phase) Escherichia coli cells increases four- to sixfold following growth in a defined or a complex medium. Chemostat-grown cells, subjected to increasing carbon starvation, also become progressively richer in sigma(S) content. These increases occur despite reduced transcription of the sigma(S)-encoding gene, rpoS, and translation of rpoS mRNA, and result solely from a large increase in the stability of the sigma protein. Previous results, based on rpoS ::lacZ transcriptional and translational fusions, and on methionine incorporation in sigma(S), had suggested increased synthesis of sigma(S) in starving cells. Alternative explanations for these results consistent with the conclusions of this paper are discussed.