In chloroplasts, this enzyme has been exclusively localized to TMs (Soll et al., 1983; Eckhardt et al., 2004;Fig. 3). If this TM localization of CS also holds for cyanobacteria, TM-synthesized chlide a could be rapidly converted to chl a, whereas chlide Bleomycin order a synthesized by the minor POR fraction in PDMs would accumulate due to scarce further processing. However, previous CS activity measurements in Synechocystis 6803 suggested the presence of CS in both the – putatively PDM-related – thylakoid centers and TMs (Hinterstoisser et al., 1993). Hence, higher chlide a synthesis rates in PDMs must also be considered. These might be due to the activity of the second, light-independent, POR enzyme (LiPOR) from Synechocystis 6803,
whose localization is still elusive (Armstrong, 1998). Taken together and despite
several open questions, the facts presented draw a picture of PDMs as a subcompartment, in which not only protein complex biogenesis but also the later steps in chlorophyll synthesis and its insertion into polypeptides occur. In conclusion, we propose the following working model for the biogenesis of TMs in the model organism Synechocystis 6803 (Fig. 4): both protein synthesis/assembly and chlorophyll synthesis/insertion are subject to tight spatial organization. These two processes are localized in a specialized membrane region, here termed PDMs, which is marked by the D1-bound form of the PSII biogenesis factor PratA. The fact learn more that non-D1-bound PratA is a soluble periplasmic protein strongly argues for at least temporary contacts of PDMs with the PM. These areas of contact are likely to be identical to the previously described thylakoid centers, which are located at the cell periphery, between PM and TMs (Hinterstoisser et al., 1993; van de Meene et al., 2006). Thiamine-diphosphate kinase Hence, the existence of such structures close to both the PM and the TM could easily explain the involvement of the periplasmic PratA factor in TM biogenesis. Furthermore, the finding that pD1, Pitt and POR are all localized
to a higher amount in PDMs upon inactivation of PratA strongly suggests an essential role of PratA in the functional and/or structural organization of these biogenesis centers and, thus, membrane flow from PDMs to TMs. Although the described model seems to apply to PSII biogenesis, less evidence is available concerning the spatial organization of the PSI assembly process. Nevertheless, the detection of the PSI reaction center proteins PsaA and PsaB in PM or PM-related fractions suggests that also PSI biogenesis is initiated in the PM or even in PDMs similar to PSII (Zak et al., 2001). Future work will be directed toward the visualization of the biogenesis process, for instance by time-resolved studies with green fluorescent protein-tagged proteins. The ultrastructural localization of the various factors involved, especially the PratA protein, will unambiguously answer the question whether, indeed, PDMs and thylakoid centers are directly linked.