EVIDENCE FOR SELF ASSOCIATION OF THE ALTERNATIVE SIGMA FACTOR SIGMA54 OF PSEUDOMONAS PUTIDA

Sigma factor σ54 has a distinct modus operandi for mediating activation of bacterial RNA polymerase (RNAP) at promoter recognition motifs 12 and 24 base pairs upstream from transcription start sites. For all steps in transcription, σ54 was thought to act as monomer. We now provide evidences that σ54 of Pseudomonas putida stably interacts with itself. The self-self interface amid monomers involves contacts in σ54 regions I and III, sequences which play key roles in the transcription activation of σ54-RNAP holoenzyme, including interactions with activator proteins and the -12 and -24 motifs. In particular, we detected inter-monomer contacts between region I, and between region I and the C-terminal portion of region III. Our results suggest new regulatory states of σ54. These may function independently of core RNAP, as a canonical transcription factor, and/or as auto-antagonist in the competition of other σ factors for binding to core RNAP after each round of RNA synthesis.