Transcription activation by Escherichia coli catabolite activator protein (CAP) at each of two classes of simple CAP-dependent promoters is understood in structural and mechanistic detail. At class I CAP-dependent promoters, CAP activates transcription from a DNA site located upstream of the DNA site for RNA polymerase holoenzyme (RNAP); at these promoters, transcription activation involves protein-protein interactions between CAP and the RNAP alpha subunit C-terminal domain that facilitate binding of RNAP to promoter DNA to form the RNAP-promoter closed complex. At class II CAP-dependent promoters, CAP activates transcription from a DNA site that overlaps the DNA site for RNAP; at these promoters, transcription activation involves both: (i) protein-protein interactions between CAP and RNAP alpha subunit C-terminal domain that facilitate binding of RNAP to promoter DNA to form the RNAP-promoter closed complex; and (ii) protein-protein interactions between CAP and RNAP alpha subunit N-terminal domain that facilitates isomerization of the RNAP-promoter closed complex to the RNAP-promoter open complex. Straightforward combination of the mechanisms for transcription activation at class I and class II CAP-dependent promoters permits synergistic transcription activation by multiple molecules of CAP, or by CAP and other activators. Interference with determinants of CAP or RNAP involved in transcription activation at class I and class II CAP-dependent promoters permits "anti-activation" by negative regulators. Basic features of transcription activation at class I and class II CAP-dependent promoters appear to be generalizable to other activators.