Biogenic membranes of the chloroplast in Chlamydomonas reinhardtii

Abstract

The polypeptide subunits of the photosynthetic electron transport complexes in plants and algae are encoded by two genomes. Nuclear genome-encoded subunits are synthesized in the cytoplasm by 80S ribosomes, imported across the chloroplast envelope, and assembled with the subunits that are encoded by the plastid genome. Plastid genome-encoded subunits are synthesized by 70S chloroplast ribosomes directly into membranes that are widely believed to belong to the photosynthetic thylakoid vesicles. However, in situ evidence suggested that subunits of photosystem II are synthesized in specific regions within the chloroplast and cytoplasm of Chlamydomonas. Our results provide biochemical and in situ evidence of biogenic membranes that are localized to these translation zones. A "chloroplast translation membrane" is bound by the translation machinery and appears to be privileged for the synthesis of polypeptides encoded by the plastid genome. Membrane domains of the chloroplast envelope are located adjacent to the cytoplasmic translation zone and enriched in the translocons of the outer and inner chloroplast envelope membranes protein import complexes, suggesting a coordination of protein synthesis and import. Our findings contribute to a current realization that biogenic processes are compartmentalized within organelles and bacteria.photosynthesis | protein trafficking | assembly | cell | mRNA

The BSD2 ortholog in Chlamydomonas reinhardtii is a polysome‐associated chaperone that co‐migrates on sucrose gradients with the rbcL transcript encoding the Rubisco large subunit

“…In such a case, they attract chaperones in the chloroplast, including BSD2, to prevent the formation of premature S-S bonds in the RbcL chain, which must refold after its translocation; however, in the absence of BSD2, the imported RbcL chains would not necessarily recapitulate the folding events that occur during the natural synthesis of the chloroplast-encoded RbcL. This premise is in line with the recent finding that subunits of photosynthetic complexes that are encoded by the nuclear and chloroplast genomes are translated on membrane-associated 'translation zones' that are located on both sides of the chloroplast envelope (Schottkowski et al, 2012).…”

Genome-wide analysis of thylakoid-bound ribosomes in maize reveals principles of cotranslational targeting to the thylakoid membrane

“…In situ assays in Chlamydomonas chloroplasts revealed that psbA mRNA is bound to thylakoid membranes via the nascent peptide during PSII repair, but is bound independent of translation to distinct "biogenic membranes" shortly after a dark-tolight shift (14,50). Our assays were performed 1 h into the light cycle on seedling leaf tissue at a stage with a young but assembled photosynthetic apparatus.…”

Localized control of oxidized RNA

“…We focused on the 8-oxoG immunofluorescence staining of the pyrenoid because it represents oxidized RNA and was atypical; we did not see this pattern for any of the four chloroplast mRNAs or nine chloroplast proteins that we analysed previously (not including RBCL and RBCS, which were seen in the pyrenoid) (Bohne et al, 2013;Schottkowski et al, 2012;Uniacke and Zerges, 2007, 2008). Therefore, 8-oxoG immunofluorescence staining of the pyrenoid represents the specific localization of oxidized RNA and is not due, for example, to nonspecific entry into the pyrenoid of RNA from the surrounding stroma (the chloroplast compartment analogous to the cytoplasm).…”