Abstract
Photosynthesis occurs in chloroplasts in the light, where in-organello environment is drastically changed in the day/night cycle. Since active photosynthetic components such as D1 protein are rapidly replaced with newly synthesized ones, gene expression must be regulated through sensing the environmental change. Transcription of photosynthesis genes in chloroplasts is accomplished with an eubacterial-type RNA polymerase (PEP), which requires multiple nucleus-encoded sigma factors. We have made Arabidopsis transgenic with wild-type and putative phosphorylation sites-deleted SIG1 molecules, the major species of sigma factors, to express them ectopically. The plants were labeled with [32P]orthophosphate in vivo and SIG1 was recovered with antibodies. Molecular mass of complexes associated with SIG1 was examined by sucrose density gradient centrifugation. The overall results lead to the conclusion that the phosphorylation of SIG1 with Ser/Thr protein kinase in the dark interferes with formation of active holoenzyme to transcribe the photosynthesis genes.
