PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

Characterization of a plastid sigma factor AtSIG5 responsible for psbD light-responsive transcription. Mikio Nozoe^*,1, Yoko Ishizaki¹, Yuichi Tsunoyama², Yuki Tsubokura¹, Ko Kato³, Atsuhiko Shinmyo³, Yoichi Nakahira¹, Takashi Shiina¹, ¹ Faculty of Human and Environment, Kyoto city, Japan² Radioisotope Research Center, Kyoto, Japan³ Nara Institute of Science and Technology, Nara city, Japan

ABSTRACT- Eubacterial-type plastid RNA polymerase (PEP) plays a major role in chloroplast transcription. The catalytic core of PEP is encoded by plastid genes, whereas sigma-like transcription initiation factors are nuclear encoded. In bacteria, primary sigma factors are responsible for transcription from the sigma-70 type promoters of housekeeping genes. In contrast, alternative sigma factors direct RNA polymerase to initiate transcription at specialized inducible promoters. Most of the PEP-dependent promoters are reminiscent of sigma-70 type promoters comprising of -35/-10 core promoter elements. On the other hand, psbD light-responsive promoter (psbD LRP) is a unique PEP promoter that lacks the functional -35 core promoter element and of which activity is dependent on upstream cis elements and DNA binding factors. Recently, we demonstrated that AtSIG5 is an essential sigma factor for psbD LRP activity (PNAS 2004, 101, 3304-3309). In this study, DNA array-based transcription analysis of plastid genes revealed that in addition to psbC and psbD transcripts presumably derived from the psbD LRP, limited number of transcripts including psbA and a few PEP-dependent genes were reduced in the AtSIG5 deficient mutant without affecting overall transcript pattern. We further characterized the function of AtSIG5 using over-expressors. Over expression of AtSIG5 cDNA from CaMV 35S promoter in tobacco caused an increase in psbD LRP transcripts. The activity of the psbD LRP was also enhanced in transplastomic tobacco expressing AtSIG5 exclusively in chloroplasts. These results provided further evidence that AtSIG5 is a unique plastid sigma factor, directly involved in transcription initiation at the psbD LRP and other limited promoters.

KEY WORDS: psbD, sigma factor, PEP, transcription