External Subunits

As mentioned before, the FMO- protein is intimately associated with the green sulfur bacterial photosystem and often copurifies with the photosystems in biochemical preparations. Single particle analysis of isolated photosystems shows one or two FMO trimers associated with each photosystem [47], confirming previous biochemical observations. Given the homodimeric character of the photosystem the FMO trimers should be as likely to interface with each PsaA, so a FMO-photosystem complex is likely to be composed of two PscA and two FMO trimers [45]. Each FMO contains seven BChl a [48, 49]. FMO proteins are unique and do not have an easily identifiable evolutionary precursor. Only one evolutionary scenario has been proposed; in it the FMO-protein is derived from the RC itself [50]. If this idea proves true, it suggests something very interesting-that great evolutionary pressure was exerted to make the RCs connect to the chlorosomes. An initially mutated and misfolded photosystem monomer was probably a very poor energetic couple to the chlorosome, and it should have taken a long time of refinement to fit, as an analogy, this square peg into this round hole. This evolutionary pressure may have been provided by lateral gene transfer of chlorosome functionality [51, 52] and the great benefit of coupling the RC to this enormous light-harvesting complex.

Two iron sulfur clusters, FA and FB, are housed within PscB [53]. This configuration is similar to the arrangement in the photosystem with FeS-type RC (PSI) of cyanobacteria and chloroplasts. However, both the green sulfur bacterial PscB and the cyanobacterial PsaC appear not to be closely related, and may have been acquired independently from each other.

Two cytochrome c subunits encoded by pscC are associated with each RC of Chlorobium tepidum, the most studied green sulfur bacterium. A striking feature of these cytochromes is the presence of three TMHs. These TMH not only anchor PscC to the membrane, but also anchor it to the RC, a link that is not easily disrupted during RC isolation [54]. In Chlorobium limicola, however, PscC appears less tightly associated with the RC, and photosynthetically competent RCs can be obtained that do not contain PscC [55].

There are also two subunits of PscD per RC. There is some evidence that PscD has a function on the electron acceptor side and is implicated in stabilizing PscB [55], mediating ferredoxin interaction with the PscB [56], or enhancing energy transfer from the FMO protein to the RC [57]. The function of improving electron transport at the donor side combined with a low but significant sequence similarity to PsaD may indicate that PscD and PsaD have a common origin. Additionally, this could show that PscD also had a role in stabilizing RC - FMO interaction, thereby enhancing energy transfer from the chlorosome to the photosystem [57].

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