Chloroplast RNA Synthesis and Processing

Abstract

Transcription and RNA maturation are two essential steps in gene expression. In chloroplasts, transcription is carried out by at least two biochemically and genetically separable activities, which may participate in establishing different basal expression rates for ribosomal RNAs, transfer RNAs and protein-coding genes. Because chloroplast RNA polymerases do not generally terminate transcription at sites corresponding to the 3′ termini of mature transcripts, these termini must be formed by RNA processing events. In Chlamydomonas reinhardtii chloroplasts, it appears that most or all transcript 5′-ends are also formed by RNA processing rather than by transcription initiation. Thus, RNA processing converts primary transcripts of generally unknown dimensions to the mature, accumulating transcripts. Molecular, genetic and biochemical approaches have been used to unravel the chloroplast transcription and RNA processing machinery, with the most information gained to date from the analysis of chimeric reporter genes introduced into chloroplasts by biolistic transformation. The picture painted bythese data reveals both similarities and differences between these processes in Chlamydomonas and land plants. However, some perceived differences, particularly based on the phenotypes of nuclear mutants which affect chloroplast mRNA metabolism, may reflect selection or screening procedures and thus may mask an overall congruity between gene expression mechanisms in the chloroplasts of all organisms.