Roles of Group 2 Sigma Factors in Acclimation of the Cyanobacterium Synechocystis sp PCC 6803 to Nitrogen Deficiency

摘要

Acclimation of cyanobacteria to environmental conditions is mainly controlled at the transcriptional level, and sigma factors of the RNA polymerase have a central role in this process. The model cyanobacterium italic toggle="yes"Synechocystis sp. PCC 6803 has four non-essential group 2 sigma factors (SigB, SigC, SigD and SigE) that regulate global metabolic responses to various adverse environmental conditions. Here we show that although none of the group 2 sigma factors is essential for the major metabolic realignments induced by a short period of nitrogen starvation, the quadruple mutant without any group 2 sigma factors and triple mutants missing both SigB and SigD grow slowly in BG-11 medium containing only 5% of the nitrate present in standard BG-11. These Delta sigBCDE, Delta sigBCD and Delta sigBDE strains lost PSII activity rapidly in low nitrogen and accumulated less glycogen than the control strain. An abnormally high glycogen content was detected in Delta sigBCE (SigD is active), while the carotenoid content became high in Delta sigCDE (SigB is active), indicating that SigB and SigD regulate the partitioning of carbon skeletons in low nitrogen. Long-term survival and recovery of the cells after nitrogen deficiency was strongly dependent on group 2 sigma factors. The quadruple mutant and the Delta sigBDE strain (only SigC is active) recovered more slowly from nitrogen deficiency than the control strain, and Delta sigBCDE in particular lost viability during nitrogen starvation. Nitrogen deficiency-induced changes in the pigment content of the control strain recovered essentially in 1 d in nitrogen-replete medium, but little recovery occurred in Delta sigBCDE and Delta sigBDE.