mRNA localization and thylakoid protein biogenesis in the filamentous heterocyst-forming cyanobacterium <i>Anabaena</i> sp. PCC 7120

Heterocyst-forming cyanobacteria such as <i>Anabaena</i> (<i>Nostoc</i>) sp. PCC 7120 exhibit extensive remodeling of their thylakoid membranes during heterocyst differentiation. Here we investigate the sites of translation of thylakoid membrane proteins in <i>Anabaena</i> vegetative cells and developing heterocysts, using mRNA fluorescent <i>in situ</i> hybridization (FISH) to detect the location of specific mRNA species. We probed mRNAs encoding reaction center core components and the heterocyst-specific terminal oxidases Cox2 and Cox3. As in unicellular cyanobacteria, the mRNAs encoding membrane-integral thylakoid proteins are concentrated in patches at the inner face of the thylakoid membrane system, adjacent to the central cytoplasm. These patches mark the putative sites of translation and membrane insertion of these proteins. Oxidase activity in mature heterocysts is concentrated in the specialized "honeycomb" regions of the thylakoid membranes close to the cell poles. However, <i>cox2</i> and <i>cox3</i> mRNAs remain evenly distributed over the inner face of the thylakoids, implying that oxidase proteins migrate extensively after translation to reach their destination in the honeycomb membranes. The RNA-binding protein RbpG is the closest <i>Anabaena</i> homolog of Rbp3 in the unicellular cyanobacterium <i>Synechocystis</i> sp. PCC 6803, which we previously showed to be crucial for the correct location of photosynthetic mRNAs. An <i>rbpG</i> null mutant shows decreased cellular levels of photosynthetic mRNAs and photosynthetic complexes, coupled with perturbations to thylakoid membrane organization and lower efficiency of the Photosystem II repair cycle. This suggests that the chaperoning of photosynthetic mRNAs by RbpG is important for the correct coordination of thylakoid protein translation and assembly.IMPORTANCECyanobacteria have a complex thylakoid membrane system which is the site of the photosynthetic light reactions as well as most of the respiratory activity in the cell. Protein targeting to the thylakoids and the spatial organization of thylakoid protein biogenesis remain poorly understood. Further complexity is found in some filamentous cyanobacteria that produce heterocysts, specialized nitrogen-fixing cells in which the thylakoid membranes undergo extensive remodeling. Here we probe mRNA locations to reveal thylakoid translation sites in a heterocyst-forming cyanobacterium. We identify an RNA-binding protein important for the correct co-ordination of thylakoid protein translation and assembly, and we demonstrate the effectiveness of mRNA fluorescent <i>in situ</i> hybridization (FISH) as a way to probe cell-specific gene expression in multicellular cyanobacteria.