Mutant forms of Salmonella typhimurium sigma(54) defective in transcription initiation but not promoter binding activity

Transcription initiation with sigma(54)-RNA polymerase holoenzyme (sigma(54 )-holoenzyme) has absolute requirements for an activator protein and ATP hy drolysis. sigma(54)'s binding to core RNA polymerase and promoter DNA has b een well studied, but little is known about its role in the subsequent step s of transcription initiation. Following random mutagenesis, we isolated ei ght mutant forms of Salmonella typhimurium sigma(54) that were deficient in transcription initiation but still directed sigma(54)-holoenzyme to the pr omoter to form a closed complex, Four of these mutant proteins had amino ac id substitutions in region I, which had been shown previously to be require d for sigma(54)-holoenzyme to respond to the activator. From the remaining mutants, we identified four residues in region In which when altered affect the function of sigma(54) at some point after closed-complex formation, Th ese results suggest that in addition to its role in core and DNA binding, r egion III participates in one or more steps of transcription initiation tha t follow closed-complex formation.