Transcription initiation-defective forms of sigma(54) that differ in ability to function with a heteroduplex DNA template

Transcription by sigma (54)-RNA polymerase holoenzyme requires an activator that catalyzes isomerization of the closed promoter complex to an open com plex. We examined mutant forms of Salmonella enterica serovar Typhimurium s igma (54) that were defective in transcription initiation but retained core RNA polymerase- and promoter-binding activities. Four of the mutant protei ns allowed activator-independent transcription from a heteroduplex DNA temp late. One of these mutant proteins, L124P V148A, had substitutions in a seq uence that had not been shown previously to participate in the prevention o f activator-independent transcription. The remaining mutants did not allow efficient activator-independent transcription from the heteroduplex DNA tem plate and had substitutions within a conserved 20-amino-acid segment (Leu-1 79 to Leu-199), suggesting a role for this sequence in transcription initia tion.