The general stress-induced sigma subunit sigma s of Escherichia coli RNA polymerase is closely related to the vegetative sigma factor sigma 70. In view of their very similar promoter specificity in vitro, it is unclear how sigma factor selectivity in the expression of sigma s-dependent genes is generated in vivo. The csiD gene is such a strongly sigma s-dependent gene. In contrast to sigma s, which is induced in response to many different stresses, csiD, whose expression is driven from a single promoter, is induced by carbon starvation only. To our knowledge, the csiD promoter is the first characterized promoter which is not only exclusively dependent on sigma s-containing RNA polymerase (E sigma s), but also requires an activator, cAMP-CRP. In addition, leucine-responsive regulatory protein (Lrp) acts as a positive modulator of csiD expression. Also in vitro, E sigma s is more efficient than E sigma 70 in csiD promoter binding, open complex formation and run-off transcription, which might be due to the poor match of the csiD -35 region to the sigma 70 consensus and to transcription by E sigma s being less dependent on contacts in this region. By DNase I protection experiments, a cAMP-CRP binding site centered at -68.5 nucleotides upstream of the csiD transcriptional start site was identified. While cAMP-CRP stimulates E sigma 70 binding, it does not promote open complex formation by E sigma 70, but does so in conjunction with E sigma s. With linear templates, cAMP-CRP significantly stimulates E sigma s-mediated in vitro transcription, whereas transcription by E sigma 70 is negligible and hardly stimulated by cAMP-CRP. These findings may reflect different or less stringent positional requirements for an activator site for E sigma s than for E sigma 70, and indicate that cAMP-CRP contributes to sigma factor selectivity at the csiD promoter. In vitro transcription experiments with super-coiled templates, however, revealed significant cAMP-CRP-stimulated transcription also by E sigma 70. Yet, under these conditions, H-NS was found to restore E sigma s specificity by strongly interfering with cAMP-CRP/E sigma 70-dependent transcription. Lrp strongly and cooperatively binds to multiple sites located between positions -14 and -102 (in a way that suggests DNA wrapping around multiple Lrp molecules) and moderately stimulates in vitro transcription, especially with E sigma s. In summary, we conclude that the csiD promoter has an intrinsic preference for E sigma s, but that also protein factors such as cAMP-CRP, Lrp and probably H-NS as well as DNA conformation contribute to its strong E sigma s selectivity. Furthermore, this strong E sigma s preference in combination with a requirement for high concentrations of the essential activator cAMP-CRP ensures csiD expression under conditions of carbon starvation, but not other stress conditions.