mRNA localisation, reaction centre biogenesis and thylakoid membrane targeting in cyanobacteria

The thylakoid membranes of cyanobacteria form a complex intracellular membrane system that has been inherited, with modifications, in chloroplasts. Cyanobacterial thylakoids are the sole site of photosynthetic electron transport and the major site of respiration, and they have a proteome that is sharply distinct from that of the cytoplasmic membrane. The sites of protein biogenesis and the mechanism that targets specific proteins to a specific cyanobacterial membrane have never been resolved. We have been trying to get at these questions by tracking mRNAs, using single-molecule Fluorescent in situ Hybridisation to probe their subcellular location in several species of cyanobacteria. The mRNAs encoding the core subunits of the photosystems cluster in tight foci at thylakoid surfaces mainly adjacent to the central cytoplasm and the nucleoid, while other mRNAs have different locations.Inhibitor studies show that this pattern of localisation is influenced by mRNA association with the ribosome, and active translation. Therefore, the foci represent the major sites of translation and membrane integration of the core reaction centre proteins. However, the mRNAs still stick to the innermost thylakoid membrane layer even after detachment from the ribosomes with puromycin. We therefore suspect that the primary targeting signal controlling the eventual destination of the protein is in the mRNA rather than the polypeptide. We have identified two RNA-binding proteins that strongly influence the location of thylakoid mRNAs and the composition of the photosynthetic apparatus.

Mahbub M, Hemm L, Yang Y, Kaur R, Carmen H, Engl C, Huokko T, Riediger M, Watanabe S, Liu L-N, Wilde A, Hess WR, Mullineaux CW (2020) mRNA localisation, reaction centre biogenesis and thylakoid membrane targeting in cyanobacteria. Nature Plants in press