Practice question

Sigma factor constitutes dissociable specificity subunit converting promoter-non-specific core polymerase into holoenzyme able to recognize -10 and -35 promoter elements and initiate transcription at defined sites. During initiation, sigma domains 2-4 contact DNA, facilitate melting, and stabilize open complex. After RNA chain reaches about ten nucleotides, sigma releases from core, reducing affinity for promoter and allowing transition to elongation. Core continues elongation independently. Housekeeping sigma70 used exponentially, alternative sigmas redirect polymerase to stress promoters. Therefore sigma primarily governs initiation step, not elongation, termination, or proofreading functions which are core or accessory factor mediated.