Arabidopsis BPG2: a phytochrome-regulated gene whose protein product binds to plastid ribosomal RNAs

BPG2 (Brz-insensitive pale green 2) is a dark-repressible and light-inducible gene that is required for the greening process in Arabidopsis. Light pulse experiments suggested that light-regulated gene expression of BPG2 is mediated by phytochrome. The T-DNA insertion mutant bpg2-2 exhibited a reduced level of chlorophyll and carotenoid pigmentation in the plastids. Measurements of time resolved chlorophyll fluorescence and of fluorescence emission at 77 K indicated defective photosystem II and altered photosystem I functions in bpg2 mutants. Kinetic analysis of chlorophyll fluorescence induction suggested that the reduction of the primary acceptor (QA) is impaired in bpg2. The observed alterations resulted in reduced photosynthetic efficiency as measured by the electron transfer rate. BPG2 protein is localized in the plastid stroma fraction. Co-immunoprecipitation of a formaldehyde cross-linked RNA-protein complex indicated that BPG2 protein binds with specificity to chloroplast 16S and 23S ribosomal RNAs. The direct physical interaction with the plastid rRNAs supports an emerging model whereby BPG2 provides light-regulated ribosomal RNA processing functions, which are rate limiting for development of the plastid and its photosynthetic apparatus.