Chloroplast gene expression-RNA synthesis and processing

Abstract

Both transcription and transcript processing are more complex in chloroplasts than in bacteria. Plastid genes are transcribed by a plastid-encoded RNA polymerase (PEP) and one (monocots) or two (dicots) nuclear-encoded RNA polymerase(s) (NEP). PEP is a bacterial-type multisubunit enzyme composed of core subunits (coded for by the plastid rpoA, B, C1 and C2 genes) and additional protein factors encoded in the nuclear genome. The nuclear genome of Arabidopsis contains six genes for sigma factors required by PEP for promoter recognition. NEP activity is represented by phage-type RNA polymerases. Factors supporting NEP activity have not been identified yet. NEP and PEP use different promoters. Both types of RNA polymerase are active in proplastids and all stages of chloroplast development. PEP is the dominating transcriptase in chloroplasts. Chloroplast RNA processing consists of hundreds of mostly independent events. In recent years, much progress has been made in identifying factors behind RNA splicing and RNA editing. Namely, pentatricopeptide repeat (PPR) proteins have come into focus as RNA binding proteins conferring specificity to individual processing events. Also, studies on chloroplast RNases have helped considerably to understand chloroplast RNA turnover. Such mechanistic insights are set in contrast to how little we know about the regulatory role of RNA processing in chloroplasts.