Transcription from the TOL plasmid Pm promoter is dependent on the XylS regulator activated by benzoate effectors. We analysed transcription from Pm in several backgrounds with differing Escherichia coli alpha and sigma subunits of RNA polymerase. In different RpoA backgrounds, transcription from Pm was as high as in the wild-type background throughout the growth curve. In the sigma S-deficient background provided by E. coli RH90, high levels of transcription from Pm (XylS/3-methylbenzoate dependent) were observed in the early logarithmic growth phase but not in the late logarithmic phase or early stationary phase. This contrasted with the results obtained in the isogenic sigma S-proficient background, in which high levels of transcription were observed throughout the growth curve. XylS/3-methylbenzoate-dependent transcription from Pm in the late logarithmic growth phase in the RH90 background was restored by cloned rpoS. The transcription initiation point of Pm was the same regardless of the growth phase and the sigma S background. The requirement of sigma S for stimulation of transcription from Pm in the late logarithmic and early stationary phase was overcome by using certain mutant Pm promoters, e.g. Pm5 (C-47-->G, A-44-->G), and the mutant regulator XylSG44S. It is suggested that the transcription from Pm involves the use of two sigma factors: sigma 70 during the early logarithmic phase and sigma S thereafter.