ChlH, the H subunit of the Mg-chelatase, is an anti-sigma factor for SigE in Synechocystis sp. PCC 6803

Although regulation of sigma factors has been intensively investigated, anti-sigma factors have not been identified in oxygenic photosynthetic organisms. A previous study suggested that the sigma factor, SigE, of the cyanobacterium Synechocystis sp. PCC 6803, a positive regulator of sugar catabolism, is posttranslation-ally activated by light-to-dark transition. In the present study, we found that the Hsubunit of Mg-chelataseChlH interacts with sigma factor SigE by yeast two-hybrid screening, and immunoprecipita-tion analysis revealed that ChlH associates with SigE in a light-dependent manner in vivo. We also found that Mg²⁺ promotes the interaction of SigE and ChlH and determines their localization in vitro. In vitro transcription analysis demonstrated that ChlH inhibits the transcription activity of SigE. Based on these results, we propose a model in which ChlH functions as an anti-sigma factor, transducing light signals to SigE in a process mediated by Mg ²⁺.