Regulation of hutUH operon expression by the catabolite gene activator protein-cyclic AMP complex in Klebsiella aerogenes.

RNA polymerase transcribed the hutUH operon of Klebsiella aerogenes if the catabolite gene activator protein (CAP) and cyclic AMP (cAMP) were present or if the DNA template was derived from a promoter mutant in which hutUH expression was independent of the need for positive effectors. In the absence of CAP or cAMP, not only was hutUH transcription absent, but transcription in the opposite direction (toward hutC) was initiated at a site (pC) ca. 70 base pairs from the site (pUH) of hutUH mRNA initiation. When the pC promoter was cloned in front of a promoterless galK gene, active expression of galK was observed. Thus, the pC promoter is active in vivo as well as in vitro. Transcription from pUH and pC may be mutually exclusive, with the major effect of CAP and cAMP being to prevent transcription from pC, thus relieving the antagonistic effect on transcription from pUH. This "double-negative" control by CAP-cAMP is supported by two observations: (i) CAP-cAMP was unable to activate transcription from pUH if RNA polymerase had been previously bound to pC and (ii) a mutation that allowed transcription from pUH in the absence of positive effectors simultaneously eliminated the activity of pC. An alternative model, in which CAP-cAMP is required for pUH expression and RNA polymerase binding at pC serves to modulate this control in some unknown way, is also considered. The physiological role of the transcript from pC other than regulation of pUH is unknown.