The Ada protein is a class I transcription factor of Escherichia coli.

The methylated Ada protein of Escherichia coli, a regulatory protein for the adaptive response, binds to a target DNA from positions -62 to -31 upstream of the ada gene and facilitates the binding of RNA polymerase to the promoter. Mutant RNA polymerases consisting of C-terminal-deleted alpha subunits are virtually inactive in response to activation by the Ada protein. Thus, we conclude that the Ada protein is a class I transcription factor which requires the C-terminal region of the RNA polymerase alpha subunit for transcription activation.