Studies on factors involved in post-transcriptional steps in plastids

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Five RNA-binding proteins (RNPs) have been isolated from tobacco chloroplasts, and their cDNAs and genes were analyzed. All five RNPs contain each a transit-peptide at the N-terminus, an acidic domain, and two RNA-binding domains towards the C-terminus. Their genes consist of four exons and three introns, the position of which is identical among the five genes.All five RNPs, though showing strong affinity to poly (G) and poly (U), bind to all plastid RNAs examined except mature tRNAs, suggesting little RNA sequence specificity. After removing all RNPs by their antibodies from chloroplast lysates, plastid RNA processing and translation activities increased. This result suggests that chloroplast RNPs function to protect and store RNAs, or to partition plastid transcription and translation systems.