SummaryThe transport and sorting of extracytoplasmic proteins in cyanobacteria is made complex by the presence of a highly differentiated membrane system. Proteins destined for the periplasm and thylakoid lumen are initially transported by Sec-and Tat-type pathways but little is known of the mechanisms that ultimately direct them to the correct destinations. We have generated a Synechocystis PCC6803 transformant that expresses a fusion protein comprising the Tat-specific targeting signal of Escherichia coli TorA linked to green fluorescent protein (GFP). Immunoblotting indicates the presence of mature-size GFP but no precursor form, demonstrating that efficient translocation has taken place. Confocal microscopy and immunogold electron microscopy reveal GFP to be almost exclusively located in the periplasm, with almost no protein evident in the thylakoid network. These data point to the operation of highly effective sorting pathways for soluble proteins in this cyanobacterium. The observed sorting of the GFP suggests that either (a) the Tat apparatus is located only in the plasma membrane or (b) the TorA-GFP is targeted across either membrane but the GFP is subsequently directed to the periplasm, perhaps by a default sorting pathway to this compartment.
BackgroundS-PM2 is a phage capable of infecting strains of unicellular cyanobacteria belonging to the genus Synechococcus. S-PM2, like other myoviruses infecting marine cyanobacteria, encodes a number of bacterial-like genes. Amongst these genes is one encoding a MazG homologue that is hypothesized to be involved in the adaption of the infected host for production of progeny phage.Methodology/Principal FindingsThis study focuses on establishing the occurrence of mazG homologues in other cyanophages isolated from different oceanic locations. Degenerate PCR primers were designed using the mazG gene of S-PM2. The mazG gene was found to be widely distributed and highly conserved among Synechococcus myoviruses and podoviruses from diverse oceanic provinces.Conclusions/SignificanceThis study provides evidence of a globally connected cyanophage gene pool, the cyanophage mazG gene having a small effective population size indicative of rapid lateral gene transfer despite being present in a substantial fraction of cyanophage. The Prochlorococcus and Synechococcus phage mazG genes do not cluster with the host mazG gene, suggesting that their primary hosts are not the source of the mazG gene.
The parent core structure of mycosporine-like amino acids (MAAs) is 4-deoxygadusol, which, in cyanobacteria, is derived from conversion of the pentose phosphate pathway intermediate sedoheptulose 7-phosphate by the enzymes 2-epi-5-epivaliolone synthase (EVS) and O-methyltransferase (OMT). Yet, deletion of the EVS gene from Anabaena variabilis ATCC 29413 was shown to have little effect on MAA production, thus suggesting that its biosynthesis is not exclusive to the pentose phosphate pathway. Herein, we report how, using pathway-specific inhibitors, we demonstrated unequivocally that MAA biosynthesis occurs also via the shikimate pathway. In addition, complete in-frame gene deletion of the OMT gene from A. variabilis ATCC 29413 reveals that, although biochemically distinct, the pentose phosphate and shikimate pathways are inextricably linked to MAA biosynthesis in this cyanobacterium. Furthermore, proteomic data reveal that the shikimate pathway is the predominate route for UV-induced MAA biosynthesis.
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