Coexpressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts.

In photosynthetic eukaryotes, thousands of proteins are translated in the cytosol and imported into the chloroplast through the concerted action of two translocons-termed TOC and TIC-located in the outer and inner membranes of the chloroplast envelope, respectively. The degree to which the molecular composition of the TOC and TIC complexes is conserved over phylogenetic distances has remained controversial. Here, we combine transcriptomic, biochemical, and genetic tools in the green alga Chlamydomonas () to demonstrate that, despite a lack of evident sequence conservation for some of its components, the algal TIC complex mirrors the molecular composition of a TIC complex from The Chlamydomonas TIC complex contains three nuclear-encoded subunits, Tic20, Tic56, and Tic100, and one chloroplast-encoded subunit, Tic214, and interacts with the TOC complex, as well as with several uncharacterized proteins to form a stable supercomplex (TIC-TOC), indicating that protein import across both envelope membranes is mechanistically coupled.

Chloroplast Biogenesis Controlled by DELLA-TOC159 Interaction in Early Plant Development.

Chloroplast biogenesis, visible as greening, is the key to photoautotrophic growth in plants. At the organelle level, it requires the development of non-photosynthetic, color-less proplastids to photosynthetically active, green chloroplasts at early stages of plant development, i.e.

Conditional Depletion of the Chlamydomonas Chloroplast ClpP Protease Activates Nuclear Genes Involved in Autophagy and Plastid Protein Quality Control.

Plastid protein homeostasis is critical during chloroplast biogenesis and responses to changes in environmental conditions. Proteases and molecular chaperones involved in plastid protein quality control are encoded by the nucleus except for the catalytic subunit of ClpP, an evolutionarily conserved serine protease. Unlike its Escherichia coli ortholog, this chloroplast protease is essential for cell viability.

Repression of essential chloroplast genes reveals new signaling pathways and regulatory feedback loops in chlamydomonas.

Although reverse genetics has been used to elucidate the function of numerous chloroplast proteins, the characterization of essential plastid genes and their role in chloroplast biogenesis and cell survival has not yet been achieved. Therefore, we developed a robust repressible chloroplast gene expression system in the unicellular alga Chlamydomonas reinhardtii based mainly on a vitamin-repressible riboswitch, and we used this system to study the role of two essential chloroplast genes: ribosomal protein S12 (rps12), encoding a plastid ribosomal protein, and rpoA, encoding the α-subunit of chloroplast bacterial-like RNA polymerase. Repression of either of these two genes leads to the arrest of cell growth, and it induces a response that involves changes in expression of nuclear genes implicated in chloroplast biogenesis, protein turnover, and stress.

Identification of an OPR protein involved in the translation initiation of the PsaB subunit of photosystem I.

Genetic analysis of mutants deficient in the biosynthesis of the photosystem I complex has revealed several nucleus-encoded factors that act at different post-transcriptional steps of chloroplast gene expression. Here we have identified and characterized the gene affected in the tab 1-F15 mutant, which is specifically deficient in the translation of the photosystem I reaction center protein PsaB as the result of a single nucleotide deletion. This gene encodes Tab 1, a 1287 amino acid protein that contains 10 tandem 38-40 amino acid degenerate repeats of the PPPEW/OPR (octatricopeptide repeat) family, first described for the chloroplast translation factor Tbc2.