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Pseudomonas oleovorans GPo1 can metabolize medium-chain-length alkanes by means of an enzymatic system whose induction is regulated by the AlkS protein. In the presence of alkanes, AlkS activates the expression of promoter PalkB, from which most of the genes of the pathway are transcribed. In addition, expression of the first enzyme of the pathway, alkane hydroxylase, is known to be influenced by the carbon source present in the growth medium, indicating the existence of an additional overimposed level of regulation associating expression of the alk genes with the metabolic status of the cell. Reporter strains bearingPalkB-lacZ transcriptional fusions were constructed to analyze the influence of the carbon source on induction of thePalkB promoter by a nonmetabolizable inducer. Expression was most efficient when cells grew at the expense of citrate, decreasing significantly when the carbon source was …