The omega subunit of the RNA polymerase core directs transcription efficiency in cyanobacteria

The catalytic core of the RNA polymerase of most eubacteria is composed of two α subunits and β, β’ and ω subunits. In Escherichia coli, the ω subunit (encoded by the rpoZ gene) has been suggested to assist β’ during RNA polymerase core assembly. The function of the ω subunit is particularly interesting in cyanobacteria because the cyanobacterial β’ is split to N-terminal γ and C-terminal β’ subunits. The ∆rpoZ strain of the cyanobacterium Synechocystis sp. PCC 6803 grew well in standard conditions although the mutant cells showed low light-saturated photosynthetic activity, low Rubisco content and accumulated high quantities of protective carotenoids and α-tocopherol. The ∆rpoZ strain contained 15% less of the primary σ factor, SigA, than the control strain, and recruitment of SigA to the RNA polymerase core was inefficient in ∆rpoZ. Thus, a cyanobacterial RNA polymerase holoenzyme lacking the ω subunit contains less frequently the primary σ factor. A DNA microarray analysis revealed that this leads to specific down-regulation of highly expressed genes, like genes encoding subunits for Rubisco, ATP synthase, NADH-dehydrogenase and carbon concentrating mechanisms. On the contrary, many genes showing only low or moderate expression in the control strain were up-regulated in ∆rpoZ. A conserved -10 region was detected in promoters showing up or down-regulation in ∆rpoZ, but -35 regions of down-regulated genes completely differed from -35 regions of up-regulated genes. Cells from cyanobacteria Synechocystis sp. PCC 6803 named as control strain (CS) and RNA polymerase omega subunit inactivation strain, ΔrpoZ, were harvested (A730=1, 40 mL) directly from standard growth conditions (continuous illumination at the PPFD of 40 µmol m-2s-1, 32°C, ambient CO2). From three to four independent experiments were performed at each conditions.