Simultaneous Inactivation of Sigma Factors B and D Interferes with Light Acclimation of the Cyanobacterium Synechocystis sp. Strain PCC 6803

In cyanobacteria, gene expression is regulated mainly at the level of transcription initiation, which is mediated by the RNA polymerase holoenzyme. The RNA polymerase core is catalytically active, while the σ factor recognizes promoter sequences. Group 2 σ factors are similar to the principal σ factor but are nonessential. Group 2 σ factors SigB and SigD are structurally the most similar σ factors in Synechocystis sp. strain PCC 6803. Under standard growth conditions, simultaneous inactivation of sigB and sigD genes did not affect the growth, but the photosynthesis and growth of the ΔsigBD strain were slower than in the control strain at double light intensity. Light-saturated electron transfer rates and the fluorescence and thermoluminescence measurements showed that photosynthetic light reactions are fully functional in the ΔsigBD strain, but absorption and 77 K emission spectra measurements suggest that the light-harvesting system of the ΔsigBD strain does not acclimate normally to higher light intensity. Furthermore, the ΔsigBD strain is more sensitive to photoinhibition under bright light because impaired upregulation of psbA genes leads to insufficient PSII repair.