, 2005; Pisareva et al., 2007), most likely due to the poor solubility and low abundance of membrane proteins in general. One of the TatA/B homologues (slr1046) could be visualized in the thylakoid membranes when fused to GFP (Aldridge et al., 2008). It remained unclear whether it was also present in the plasma membrane, and currently no data are available on the localization of ssl2823. An analysis of the 25 different cyanobacterial genomes reveals the presence of putative Tat pathway components in all cases (Table 1). Each of them possesses a single
TatC Epacadostat homologue, with the only exception being Synechococcus JA-3-3Ab that interestingly has an additional truncated TatC (cya_1280). This truncated version of TatC comprises only two predicted transmembrane domains compared to the usual six found in TatC proteins and its function in the Tat pathway remains to be experimentally verified. Amongst many of the marine cyanobacteria strains, the tatC gene is localized in a relatively well-conserved cluster that includes the predicted petC Tat substrate (Fig. 1) that is a component of the Cytochrome b6-f complex. Also, noteworthy is the close localization of the csaA gene with tatC in the two Nostoc species studied, and an example of one of these (Nostoc punctiforme Vorinostat supplier ATCC29133) is shown in Fig. 1. CsaA is a translocation associated chaperone
thought to be functionally similar to E. coli SecB that is involved in the translocation of some Sec-pathway substrates; so far, it has not been shown to participate in Tat-dependent translocation. Also intriguing is the localization of a natural resistance macrophage protein (Nramp) encoding gene between the tatC and petC genes of Nostoc punctiforme ATCC29133 (Fig. 1). Nramp proteins are metal ion transporters that
have been found to transport Mn2+ and Fe2+ (Makui et al., 2000), and this close localization with PetC and TatC may suggest a role in the delivery of iron to PetC. A blast search against the sequences of the Tat proteins of Synechococcus sp. WH8102 also reveals that many of the 25 genomes analysed have just a single TatA/B Thymidine kinase homologue, strongly indicating the widespread use of minimal TatAC systems in these species, although we cannot rule out the existence of other more divergent TatA like proteins. However, some of the strains analysed do have two separate TatA/B homologues and again it remains unclear whether these cyanobacteria have TatABC or TatAC-type translocases. The presence of two separate TatA/B proteins in many of the Prochlorococcus strains is surprising as these have the smallest cyanobacterial genomes and the fewest predicted Tat substrates of the strains studied (Tables 1 and S1). One of the two tatA genes of Synechococcus JA-3-3Ab (cya_0761) is localized with genes encoding FtsY and SecA that are required for protein translocation via the signal recognition particle pathway and general secretory pathway (Du Plessis et al., 2011) respectively (Fig. 1).