Indirect effects of the 3'-5' cyclic adenosine monophosphate binding protein (CAP) on the transcription of the malPQ operon in Escherichia coli.

Uninduced malPQ transcription, as followed by measuring beta-galactosidase expression in a strain carrying a malP-lacZ hybrid gene and grown in the absence of maltose, requires the presence of CAP. However this requirement is lost when the expression of malT, positive regulator gene of the maltose regulon, is rendered independent of CAP by a mutation in the malT promoter. This result suggests that the effect of CAP on uninduced malPQ expression is mediated through a modulation of MalT protein synthesis. The effect of CAP on the induced expression of malPQ is presumably mediated, in addition, through a modulation of the synthesis of the maltose transport system and, hence, of the entry of the inducer. Therefore the effect of CAP on malPQ expression seems to be merely indirect, and this is surprising since a CAP binding site is present at the malPQ promoter.