The interaction of ω(2) with the RNA polymerase β’ subunit functions as an activation to repression switch

The ω gene is encoded in broad-host range and low-copy plasmids. It is genetically linked to antibiotic resistance genes of the major human pathogens of phylum Firmicutes. The homodimeric forms of ω (ω(2)) coordinate the plasmid copy number control, faithful partition (ω(2) and δ(2)) and better-than-random segregation (ζϵ(2)ζ) systems. The promoter (P) of the ωϵζ operon (P(ω)) transiently interacts with ω(2). Adding δ(2) facilitates the formation of stable ω(2)·P(ω) complexes. Here we show that limiting ω(2) interacts with the N-terminal domain of the β’ subunit of the Bacillus subtilis RNA polymerase (RNAP-σ(A)) vegetative holoenzyme. In this way ω(2) recruits RNAP-σ(A) onto P(ω) DNA. Partial P(ω) occupancy by ω(2) increases the rate at which RNAP-σ(A) complex shifts from its closed (RP(C)) to open (RP(O)) form. This shift increases transcription activation. Adding δ(2) further increases the rate of P(ω) transcription initiation, perhaps by stabilizing the ω(2)·P(ω) complex. In contrast, full operator occupancy by ω(2) facilitates RP(C) formation, but it blocks RP(O) isomerization and represses P(ω) utilization. The stimulation and inhibition of RP(O) formation is the mechanism whereby ω(2) mediates copy number fluctuation and stable plasmid segregation. By this mechanism, ω(2) also indirectly influences the acquisition of antibiotic resistance genes.