The XylS-dependent Pm promoter is transcribed in vivo by RNA polymerase with sigma(32) or sigma(38) depending on the growth phase

The Pm promoter of the TOL plasmid of Pseudomonas putida is expressed at hi gh level along the growth curve. This transcription is dependent on the pos itive regulator XyIS activated by 3-methylbenzoate, The sigma factor sigma( 38) is required for expression in early stationary phase and thereafter, To test whether sigma(70) was involved in Pm transcription in exponential pha se, we have followed mRNA synthesis in a rpoD thermosensitive strain, No di fference in Pm transcription was found between the wild type and the thermo sensitive strain at the restrictive temperature of 42 degrees C, indicating that transcription was independent of the sigma factor sigma(70). However, basal levels of mRNA expression from Pm in this strain in exponential phas e were more than twofold higher at 42 degrees C, suggesting involvement of sigma(32) in Pm transcription. In a rpoH background, no expression of Pm to ok place in the exponential phase, whereas it increased during stationary p hase, and in a rpoH rpoS double mutant no activity from the Pm promoter was detected along the growth curve. We have shown that the increase in the am ount of sigma(32) factor necessary for transcription in exponential phase i s provided through induction of the heat shock response by the presence of the effector 3-methylbenzoate, which is also required for activation of the positive regulator XyIS, We conclude that activation of Pm transcription i s achieved through a switch between two stress-responsive factors, sigma(32 ) in exponential phase and sigma(38) in stationary phase. In both cases, tr anscription is dependent on the activator XyIS and presents the same transc ription start point.