Chloroplast gene expression:Recent advances and perspectives

Chloroplasts evolved from an ancient cyanobacterial endosymbiont more than 1.5 billion years ago.During subsequent coevolution with the nuclear genome,the chloroplast genome has remained independent,albeit strongly reduced,with its own transcriptional machinery and distinct features,such as chloroplast-specific innovations in gene expression and complicated post-transcriptional processing.Light activates the expression of chloroplast genes via mechanisms that optimize photosynthesis,minimize photodamage,and prioritize energy investments.Over the past few years,studies have moved from describing phases of chloroplast gene expression to exploring the underlying mechanisms.In this review,we focus on recent advances and emerging principles that govern chloroplast gene expression in land plants.We discuss engineering of pentatricopeptide repeat proteins and its biotechnological effects on chloroplast RNA research;new techniques for characterizing the molecular mechanisms of chloroplast gene expression;and important aspects of chloroplast gene expression for improving crop yield and stress tolerance.We also discuss biological and mechanistic questions that remain to be answered in the future.

A Small Multifunctional Pentatricopeptide Repeat Protein in the Chloroplast of Chlamydomonas reinhardtii

Organellar biogenesis is mainly regulated by nucleus-encoded factors, which act on various steps of gene expression including RNA editing, processing, splicing, stabilization, and translation initiation. Among these regulatory factors, pentatricopeptide repeat （PPR） proteins form the largest family of RNA binding proteins, with hundreds of members in flowering plants. In striking contrast, the genome of the unicellular green alga Chlamydomonas reinhardtii encodes only 14 such proteins. In this study, we analyzed PPR7, the smallest and most highly expressed PPR protein in C. reinhardtii. Green fluorescent protein-based localization and gel-filtration analysis revealed that PPR7 forms a part of a high-molecular-weight ribonu- cleoprotein complex in the chloroplast stroma. RIP-chip analysis of PPRT-bound RNAs demonstrated that the protein associates with a diverse set of chloroplast transcripts in vivo, i.e. rrnS, psbH, rpoC2, rbcL, atpA, cemA-atpH, tscA, and atpl-psaJ. Furthermore, the investigation of PPR7 RNAi strains revealed that deple- tion of PPR7 results in a light-sensitive phenotype, accompanied by altered levels of its target RNAs that are compatible with the defects in their maturation or stabilization. PPR7 is thus an unusual type of small multi- functional PPR protein, which interacts, probably in conjunction with other RNA binding proteins, with numerous target RNAs to promote a variety of post-transcriptional events.