Abstract
Nuclear-encoding sigma factors confer promoter specificity on bacterial-type RNA polymerase in plant chloroplasts. To date, 6 SIG genes encoding sigma factor have been identified in rice and Arabidopsis. To clarify functions of one of the SIG genes, SIG1, we isolated and characterized two allelic mutants of rice SIG1, in which SIG1 is disrupted by insertion of a retrotransposon, Tos17. The mutants impaired in SIG1 were fertile, however, they showed one-third reduction of chlorophyll content in mature leaves. Semi-comprehensive analysis for chloroplast genes revealed that accumulation of psaA operon in mutant heterozygotes reduced to ~30% compared to that in wild type. The reduction of PSI complex resulted in 70% reduction of light-reaction activity. These results demonstrated that SIG1 takes significant roles in maintenance of photosynthetic activity of PSI in matured chloroplasts in leaves by regulating transcription of the genes encoding photosystem I apparatus.
