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

A nuclear-encoded sigma factor, AtSig6, plays a key role in early chloroplast development in cotyledons. Yoko Ishizaki^*,1, Yuichi Tsunoyama², Kyoko Hatano³, Kohei Ando⁴, Ko Kato⁴, Atsuhiko Shinmyo⁴, Yoichi Nakahira¹, Takashi Shiina¹, ¹ Faculty of Human and Environment, Japan² Radioisotope Research Center, Japan³ Graduate School of Human and Environmental Studies, Japan⁴ Nara Institute of Science and Technology, Japan

ABSTRACT- Plastid-encoded eubacterial-type plastid RNA polymerase (PEP) plays a major role in transcription of photosynthesis genes. PEP is composed of the plastid-encoded core enzyme and one of nuclear-encoded sigma subunits responsible for promoter recognition. Increasing evidences support the idea that the replacement of sigma factors would be the major mechanism for switching of the transcription pattern in chloroplasts as in the case of bacteria. The transcription of photosynthesis genes is known to increase transiently during the initial phase of chloroplast development to support the built-up of the photosynthesis apparatus. However, sigma factors responsible for the initial transcription of photosynthesis genes have not been identified. In recent analysis of AtSIG6 deficient mutant (sig6-1), we found that inactivation of AtSig6 led to the cotyledon-specific pale green phenotype in Arabidopsis. Light-dependent chloroplast development was significantly delayed in the sig6-1 mutants without affecting other photomorphogenic processes. Genetic complementation of the mutant phenotype by AtSIG6cDNA demonstrates that AtSig6 plays a key role in early chloroplast development in cotyledons. Northern and array-based analyses of plastid transcripts revealed that accumulation of most PEP-dependent transcripts was greatly suppressed in 4 day-old mutant seedlings, while nuclear-encoded RNA polymerase (NEP)-dependent genes were transcribed actively. Almost all AtSig6-dependent genes are preceded by sigma-70-type promoters containing conserved -35/-10 elements. These data suggest that AtSig6 may act as a general sigma factor that recognizes overall sigma-70-type PEP promoters in chloroplasts. At later stages of seedling growth, the transcriptional defects and the pale green phenotype were completely restored in the AtSIG6 deficient mutants. It is likely that another general-type sigma factor(s) might function redundantly with AtSig6 in chloroplasts of older seedlings.

KEY WORDS: PEP, sigma factor, transcription, Arabidopsis