Chloroplast translation regulation

Chloroplast gene expression is primarily controlled during the translation of plastid mRNAs. Translation is regulated in response to a variety of biotic and abiotic factors, and requires a coordinate expression with the nuclear genome. The translational apparatus of chloroplasts is related to that of bacteria, but has adopted novel mechanisms in order to execute the specific roles that this organelle performs within a eukaryotic cell. Accordingly, plastid ribosomes contain a number of chloroplast-unique proteins and domains that may function in translational regulation. Chloroplast translation regulation involves cis-acting RNA elements (located in the mRNA 5′ UTR) as well as a set of corresponding trans-acting protein factors. While regulation of chloroplast translation is primarily controlled at the initiation steps through these RNA-protein interactions, elongation steps are also targets for modulating chloroplast gene expression. Translation of chloroplast mRNAs is regulated in response to light, and the molecular mechanisms underlying this response involve changes in the redox state of key elements related to the photosynthetic electron chain, fluctuations of the ADP/ATP ratio and the generation of a proton gradient. Photosynthetic complexes also experience assembly-related autoinhibition of translation to coordinate the expression of different subunits of the same complex. Finally, the localization of all these molecular events among the different chloroplast subcompartments appear to be a crucial component of the regulatory mechanisms of chloroplast gene expression.     

Plastid biogenesis, between light and shadows

Plastids are cellular organelles which originated when a photosynthetic prokaryote was engulfed by the eukaryotic ancestor of green and red algae and land plants. Plastids have diversified in plants from their original function as chloroplasts to fulfil a variety of other roles in metabolite biosynthesis and in storage, or purely to facilitate their own transmission, according to the cell type that harbours them. Therefore cellular development and plastid biogenesis pathways must be closely intertwined. Cell biological, biochemical, and genetic approaches have generated a large body of knowledge on a variety of plastid biogenesis processes. A brief overview of the components and functions of the plastid genetic machinery, the plastid division apparatus, and protein import to and targeting inside the organelle is presented here. However, key areas in which our knowledge is still surprisingly limited remain, and these are also discussed. Chloroplast-defective mutants suggest that a substantial number of important plastid biogenesis proteins are still unknown. Very little is known about how different plastid types differentiate, or about what mechanisms co-ordinate cell growth with plastid growth and division, in order to achieve what is, in photosynthetic cells, a largely constant cellular plastid complement. Further, it seems likely that major, separate plastid and chloroplast 'master switches' exist, as indicated by the co-ordinated gene expression of plastid or chloroplast-specific proteins. Recent insights into each of these developing areas are reviewed. Ultimately, this information should allow us to gain a systems-level understanding of the plastid-related elements of the networks of plant cellular development.     

Domain of Unknown Function 143 is required for the functioning of PEP-associated protein DG238 in the chloroplast

Plastid-encoded plastid RNA polymerase (PEP) is essential for chloroplast development and plastid gene expression in Arabidopsis thaliana. However, PEP is a large complex, and many proteins in this complex remain to be identified. We previously reported that Delayed Greening 238 (DG238) interacts with PEP subunit protein FLN1 and may function as a PEP-associated protein and participate in early chloroplast development and PEP-dependent plastid gene expression. DG238 contains Domain of Unknown Function 143 (DUF143), whose function is currently unknown. Here, we found that a deficiency of the DUF143 domain in DG238 affected its localization, which resulted in abnormal interactions with PEP-associated proteins in the chloroplast. Furthermore, DG238 lacking the DUF143 domain or DG238 with only this domain failed to function. Interestingly, the lack of conserved amino acids 193–217 of the DUF143 domain in DG238 also affected its function. In addition to FLN1, DG238 also interacts with other PEP-associated proteins, including FSD2, FSD3, MRL7-L, and MRL7, to regulate plastid gene expression. These results suggest that the DUF143 domain is necessary for the functioning of the PEP-associated protein DG238 in chloroplasts.