Practice question

Escherichia coli core RNA polymerase comprises five polypeptides assembled as α2ββ'ω. Two α subunits of 36.5 kDa each provide assembly scaffold and contact UP element through C-terminal domain linker. β subunit 150.6 kDa binds rifampicin and forms part of catalytic channel. β' subunit 155.2 kDa contains conserved double-psi barrel motif NADFDGD coordinating catalytic magnesium. ω subunit 10.1 kDa chaperones β' folding and stabilizes assembly. Core alone competent for elongation but lacks promoter recognition. Association with sigma factor forms holoenzyme α2ββ'ωσ capable of specific initiation. Stoichiometry distinguishes core from holoenzyme containing sigma.