Differential transcriptomic responses of pmgA and ssr1600 single-knockout mutants to short-term and prolonged high-light exposure in the cyanobacterium Synechocystis sp. PCC 6803

PmgA and Ssr1600, homologs of the anti-sigma factor and the anti-sigma antagonist of the bacterial partner-switching system, respectively, are involved in coordination of the repression of photosystem I content and chlorophyll biosynthesis under prolonged high-light (HL) conditions in the cyanobacterium Synechocystis sp. PCC 6803. To characterize the transcriptional responses associated with PmgA and Ssr1600 during HL acclimation, we compared global gene expression profiles of the wild-type (WT) and knockout strains of pmgA (ΔpmgA) and ssr1600 (Δ1600) under low-light conditions and at 15 min, 1 h and 12 h after the shift to HL. The knockout of pmgA or ssr1600 induced temporally distinct transcriptomic alterations between short-term and prolonged HL exposure. At 15 min, Δ1600, but not ΔpmgA, showed enhanced induction of stress-responsive genes, including sigB and heat-shock genes, together with derepression of pmgA. At 12 h, both mutants showed significant upregulation of genes related to chlorophyll biosynthesis, photosynthetic electron transport, nitrogen assimilation, and amino acid biosynthesis. Considering that Ssr1600 does not accumulate in ΔpmgA, these observations can be explained by a shift in the role of Ssr1600, from the suppressor of the amplification of the stress response by PmgA under short-term HL conditions to a factor mediating signal output from the PmgA-Ssr1600 module under prolonged HL conditions, although the contribution of secondary effects cannot be excluded. Our transcriptomic data do not support the hypothesis that PmgA acts as a canonical anti-sigma factor for any of the sigma factors examined in this study, SigB, SigD and SigE, under either condition.