Genetic Engineering of Group 2 σ Factor SigE Widely Activates Expressions of Sugar Catabolic Genes in Synechocystis Species PCC 6803

Metabolic engineering of photosynthetic organisms is required for utilization of light energy and for reducing carbon emissions. Control of transcriptional regulators is a powerful approach for changing cellular dynamics, because a set of genes is concomitantly regulated. Here, we show that overexpression of a group 2 sigma factor, SigE, enhances the expressions of sugar catabolic genes in the unicellular cyanobacterium, Synechocystis sp. PCC 6803. Transcriptome analysis revealed that genes for the oxidative pentose phosphate pathway and glycogen catabolism are induced by overproduction of SigE. Immunoblotting showed that protein levels of sugar catabolic enzymes, such as glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glycogen phosphorylase, and isoamylase, are increased. Glycogen levels are reduced in the SigE-overexpressing strain grown under light. Metabolome analysis revealed that metabolite levels of the TCA cycle and acetyl-CoA are significantly altered by SigE overexpression. The SigE-overexpressing strain also exhibited defective growth under mixotrophic or dark conditions. Thus, SigE overexpression changes sugar catabolism at the transcript to phenotype levels, suggesting a sigma factor-based engineering method for modifying carbon metabolism in photosynthetic bacteria.