Sequences within the DNA cross-linking patch of sigma 54 involved in promoter recognition, sigma isomerization, and open complex formation.

The bacterial RNA polymerase holoenzyme containing the final sigma(54) subunit functions in enhancer-dependent transcription. Mutagenesis has been used to probe the function of a sequence in the final sigma(54) DNA binding domain that includes residues that cross-link to promoter DNA. Several activities of the final sigma and holoenzyme are shown to depend on the cross-linking patch. The patch contributes to promoter binding by final sigma(54), and holoenzyme and is involved in activator-dependent final sigma isomerization. As part of the final sigma(54)-holoenzyme, some residues in the patch limit basal transcription. Other cross-linking patch sequences appear to limit activator-dependent open complex formation. Deletion of 19 residues adjacent to the cross-linking patch resulted in a holoenzyme unable to respond to activator but capable of activator-independent (bypass) transcription in vitro. Overall results are consistent with the cross-linking patch directing interactions to the -12 promoter region to set basal and activated levels of transcription.