FIG. 1.
P. putida GPo1 alkane degradation pathway. The genes are grouped into two clusters, alkBFGHJKL and alkST, both of which are regulated by the AlkS protein. In the absence of alkanes, AlkS is expressed from promoter PalkS1; AlkS acts as a repressor of this promoter, allowing for low expression levels. This promoter is recognized by σS-RNA polymerase, being active only in the stationary phase of growth. In the presence of alkanes, AlkS activates expression from the PalkB and PalkS2 promoters, generating a positive amplification loop on alkS expression. Activation of these two promoters by alkanes is strongly repressed by catabolic repression when cells grow exponentially in rich LB medium. Growth in a minimal medium containing some organic acids (lactate or succinate) as a carbon source generates a milder catabolic repression effect.