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What is the mechanism by which genes are positively regulated? How can several unlinked genes encoding related functions be regulated by a common signal? These are two of the questions which can be addressed by studying the catabolite gene activator protein (CAP). CAP responds to differences in the availability and nature of carbon sources, via variations in the intracellular concentration of cyclic AMP (cAMP). CAP, when complexed with cAMP, is a sequence-specific DNA-binding protein which activates several gene systems and represses others. It has been most extensively studied for Escherichia coli, although closely related proteins exist in other gram-negative bacteria. CAP is an important paradigm for understanding the positive control of gene expression because of the extensive genetic, biochemical, and structural studies which have provided significant insights into its mode of action. This last year has been especially important because of several recent accomplishments: the determination of the molecular structure of the CAP-DNA complex (21), the genetic identification of a solvent-exposed loop between two pleated-sheet structures as being critical for positive activation (1, 5, 6), and the suggestion that the carboxy-terminal portion of the RNA polymerase a subunit may provide a contact point for CAP (12). This review concentrates on analyzing how CAP activates lac operon expression. Although it is likely that CAP's mechanism of action will be similar to that of some other positive activator proteins, still others are likely to function quite differently. Moreover, CAP itself may act differently in different cases. As will be discussed below, CAP binds at different distances relative to the transcription start point for different systems; does this require a unique mechanism for each physical arrangement? Moreover, in some cases CAP acts through an additional "intermediate" protein, introducing another partner into the equation.

Catabolite gene activator protein activation of lac transcription