In chloroplasts of Arabidopsis thaliana, the synthesis of chlorophyll was described to occur in several plastidic subcompartments (Eckhardt et al., 2004). While early steps in synthesis, i.e. the conversion of glutamate to 5-aminolevulinic acid, occur in the chloroplast stroma, the enzymes required for later steps are associated with the inner envelope membrane or the TM (Fig. 3). These membrane-attached enzymes include the NADPH-protochlorophyllide
oxidoreductase (POR) and the chlorophyll synthase (CS), which catalyze the reduction of protochlorophyllide a (pchlide a) to chlorophyllide a (chlide a) and the subsequent generation of chl Vorinostat research buy a, respectively. Similar to the situation in higher plants, previous studies revealed that cyanobacterial chlorophyll biosynthesis also underlies a spatial organization (Peschek et al., 1989; Eckhardt et al., 2004). In Synechococcus elongatus 7942 (formerly KU-60019 molecular weight called Anacystis nidulans), pchlide a and chlide a accumulate in PM preparations and cannot be detected in the TM (Peschek et al., 1989). Moreover,
in Synechocystis 6803, highest chlorophyll precursor concentrations were found in a membrane fraction suggested to represent the abovementioned thylakoid center fraction resembling PDMs (Hinterstoisser et al., 1993). As mentioned, photosynthetic precomplexes already contain chlorophyll molecules, suggesting that not only the later steps in chlorophyll synthesis but also the insertion of pigments occur at the protein assembly sites associated with the PM or PDMs (Keren et al., 2005). Further experimental evidence for an important role of PDMs in chlorophyll Ribonucleotide reductase synthesis and insertion was recently provided by the analysis of another TPR protein from Synechocystis 6803, named Pitt (POR-interacting TPR protein). This TM protein was found to interact directly with and stabilize the light-dependent POR enzyme (Schottkowski et al., 2009b). Intriguingly, in a pratA mutant, a large proportion of both Pitt and POR was localized in PDM fractions. This is in contrast to wild-type cells, where only minor amounts
are found in PDMs and the majority is TM associated (Schottkowski et al., 2009b). Hence, these two proteins are affected by the absence of PratA in the same way as the pD1 precursor protein. Apparently, a defective PSII assembly and perturbation of membrane flow from PDMs to TMs causes the retardation of additional PSII biogenesis factors, including Pitt and POR, at the site of early PSII assembly, i.e. the PDMs. However, the question arises as to why in wild-type cells chlide a is mainly localized in the PM and/or in the thylakoid centers (Peschek et al., 1989; Hinterstoisser et al., 1993), whereas the chlide a-synthesizing enzyme POR is mainly – but not exclusively – detected in the TM (Schottkowski et al., 2009b).