A positive feedback mechanism controls expression of AlkS, the transcriptional regulator of the Pseudomonas oleovorans alkane degradation pathway

The AlkS regulator, encoded by the alkS gene of the Pseudomonas oleovorans OCT plasmid, activates the expression of a set of enzymes that allow assimi lation of alkanes. We show that the AlkS protein regulates, both negatively and positively, the expression of its own gene. In the absence of alkanes, alkS is expressed from promoter PalkS1, which is recognized by sigma(S)-RN A polymerase, and whose activity is very low in the exponential phase of gr owth and considerably higher in stationary phase. AlkS was found to downreg ulate this promoter, limiting expression of alkS in stationary phase when a lkanes were absent. In the presence of alkanes, AlkS repressed PalkS1 more strongly and simultaneously activated a second promoter for alkS, named Pal kS2, located 38 bp downstream from PalkS1. Activation of PalkS2 allowed eff icient transcription of alkS when alkanes were present. Transcription from PalkS2 was modulated by catabolite repression when cells were provided with a preferred carbon source. We propose that the expression of alkS is regul ated by a positive feedback mechanism, which leads to a rapid increase in a lkS transcription when alkanes are present. This mechanism should allow a r apid induction of the pathway, as well as a fast switch-off when alkanes ar e depleted. An improved model for the regulation of the pathway is proposed .