Poster: Transcription Regulation

Abs # 1054: Functional analysis of a blue light induced plastid sigma factor, Sig5

Presenter:	Ohba, Atsushi , a_ooba@mei.kpu.ac.jp
Authors	Ohba, Atsushi  (A)   Yasuda, Hiroyuki  (B)   Tsunoyama, Yuichi  (C)   Shiina, Takashi  (A)
Affiliations:	(A): Faculty of Human and Environment, Kyoto Prefectural University (B): Graduate School of Human and Environmental Studies, Kyoto University (C): Radioisotope research center, Kyoto University

In higher plants, the plastid eubacteria-type RNA polymerase (PEP; plastid-encoded RNA polymerase) preferentially transcribes photosynthetic genes. This enzyme is composed of plastid-encoded core subunits and a nuclear-encoded sigma factor, which presumably confer promoter-specific transcription initiation on PEP. In Arabidopsis thaliana, there are six putative plastid sigma genes (SIG1-6) homologous to bacterial group 1 and 2 sigma factors. These multiple sigma factors are considered to recognize different subsets of promoters and regulate the activity of PEP in response to the developmental stage and various environmental clues. To analyze the promoter selectivity of these factors, we over produced each of sigma factors in protoplasts using the transient expression method and measured the transcription activity of a set of plastid genes by the run-on assay. Some factors were found to selectively elevate the PEP activity for different set of plastid genes, indicating differential promoter recognition property of each sigma factor. Interestingly, the over-production of Sig5 selectively enhanced the transcription of psbA and psbD genes. The main promoter upstream of psbD is known as the blue light responsive promoter (BLRP). The effects of blue light on mRNA levels of six SIG genes were also examined. The accumulation of SIG5 transcripts was induced only by blue light, while blue and red light equally induced the accumulation of other SIG genes transcripts. Taken together, it is suggested that blue light induced expression of Sig5 is involved in the mechanism of the activation of the psbD BLRP. Recently, we identified a novel plastid sigma gene homologous to Arabidopsis SIG5 from Oryza sativa. Therefore, Sig5 may function in various plant species.