The alternative sigma factor sigma(S) (RpoS) of Escherichia coli RNA polymerase regulates the expression of stationary phase and stress-response genes. sigma(S) is also required for the transcription of the cryptic genes csgBA that encode the subunits of the curli proteins. The expression of the csgBA genes is regulated in response to a multitude of physiological signals. In stationary phase, these genes are transcribed by the sigma(S) factor, and expression of the operon is enhanced by the small protein Crl. It has been shown that Crl stimulates the activity of sigma(S), leading to an increased transcription rate of a subset of genes of the rpoS regulon in stationary phase. However, the underlying molecular mechanism has remained elusive. We show here that Crl interacts directly with sigma(S) and that this interaction promotes binding of the sigma(S) holoenzyme (Esigma(S)) to the csgBA promoter. Expression of Crl is increased during the transition from growing to stationary phase. Crl accumulates in stationary phase cells at low temperature (30 degrees C) but not at 37 degrees C. We therefore propose that Crl is a second thermosensor, besides DsrA, controlling sigma(S) activity.