Poster: Organelle Biogenesis

Abs # 1139: An RNase R family protein functions in ribosomal RNA processing in chloroplasts of Arabidopsis

Presenter:	Kishine, Masahiro , kishine@kais.kyoto-u.ac.jp
Authors	Kishine, Masahiro  (A)   Takabayashi, Atsushi  (A)   Munekage, Yuri  (B)   Shikanai, Toshiharu  (B)   Endo, Tsuyoshi  (A)   Sato, Fumihiko  (A)
Affiliations:	(A): Graduate School of Biostudies, Kyoto University (B): Graduate School of Biological Science, Nara Institute of Science and Technology

Gene expression of chloroplast is dependent on a large numbers of nuclear-encoded factors. To determine such nuclear-encoded factors for chloroplast gene expression, we isolated high chlorophyll fluorescence mutants of Arabidopsis. Among 59 mutants, a mutant rnr1 was identified on the basis of high Fo (minimum yield of chlorophyll fluorescence) phenotype. The mutation was due to a T-DNA insertion affecting the expression of a gene encoding a RNase R family protein which has a putative targeting signal to chloroplasts. Since RNase R family members, represented by E coli RNase II and RNase R, have 3' to 5' exo-ribonucleae activity, we examined the chloroplast transcript amounts and profile. Whereas most plastome transcripts did not differ in size or steady state level between rnr1 and wild-type, the rRNA and ndhCKJ transcripts showed differences; rnr1 showed the impaired intercistronic cleavage between 23S and 4.5S ribosomal RNA, and the significant reduction in chloroplastic rRNA, suggesting the function in the maturation of chloroplast rRNA. In vivo translation assays demonstrated that the rate of translation of thylakoid proteins was slightly reduced with the reduced ribosomal RNA in rnr1. We also detected higher expression of some NEP dependent gene transcripts in rnr1 than in the wild type, which is possibly due to the secondary effects of the reduced rRNA levels. Ectopic expression of RNR1 in mutant reverted the phenotype. Our results indicated that the general reduction in the translation activity in chloroplast in rnr1 mutant induced the high chlorophyll fluorescence, the pale green leaf color, and the drastic reduction in growth rate.