Functional proteomics reveals that Slr0237 is a SigE-regulated glycogen debranching enzyme pivotal for glycogen breakdown

The group 2 σ factor for RNA polymerase SigE plays important role in regulating central carbon metabolism in cyanobacteria. However, the regulation of SigE for these pathways at a proteome level remains unknown. Using a sigE-deficient strain (ΔsigE) of Synechocystis sp. PCC 6803 and quantitative proteomics, we found that SigE depletion induces differential protein expression for sugar catabolic pathways including glycolysis, oxidative pentose phosphate (OPP) pathway, and glycogen catabolism. Two glycogen debranching enzyme homologues Slr1857 and Slr0237 are found differentially expressed in ΔsigE. Glycogen determination indicated that Δslr0237 accumulated glycogen under photomixotrophic conditions but was unable to utilize these reserves in the dark, whereas Δslr1857 accumulates and utilize glycogen in a similar way as the WT strain does in the same conditions. These results suggest that Slr0237 plays the major role as the glycogen debranching enzyme in Synechocystis. To our knowledge, this is the first study to report the functional difference of two glycogen debranching enzyme in Synechocystis and the research highlights the intricate regulation of glycogen breakdown.