PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Antenna ring around trimeric Photosystem I in a photosynthetic prokaryote Prochlorothrix hollandica. Ladislav Bumba^{*,1, 3}, Ondrej Prasil², Frantisek Vacha^{2, 3}, ¹ Faculty of Biological Sciences, Ceske Budejovice, Czech republic³ Institute of Plant Molecular Biology, Ceske Budejovice, Czech republic² Institute of Physical Biology, Nove Hrady, Czech republic

ABSTRACT- Three known species of oxychlorobacteria (Prochloron didemni, Prochlorothrix hollandica and Prochlorococcus marinus) are an unusual group of prokaryotes which perform oxygenic photosynthesis. They are apparently related to cyanobacteria but they lack water-soluble phycobilisomes while they synthesize chlorophyll b and to form an intrinsic chl-a/b light-harvesting antenna. The chla/b-binding proteins (encoded by pcb genes) are not closely related to the lhc genes encoding a superfamily of the light-harvesting antenna proteins of green plastids, but instead are similar to the iron-stress induced isiA gene of cyanobacteria and to CP43, a chla-antenna protein of photosystem II. In P. holandica three Pcb antenna proteins (pcbA-C gene products) have been detected. Here we show how the antenna proteins structurally interact with the photosystem I (PSI) reaction center in P. holandica. The isolated thylakoid membranes of P. hollandica were solubilized by mild detergent n-dodecyl-beta-D-maltoside and subjected to a sucrose density gradient centrifugation. On the basis of protein composition and spectroscopic data the lowest green fraction of sucrose gradient contained polypeptides of the PSI reaction center complex associated with the Pcb proteins. In order to reveal how the PSI complex interact with the Pcb proteins the fraction was negatively stained with uranyl acetate, subjected to electron microscopy and obtained images processed with SPIDER image analysis package. Single particle analysis showed a Pcb-PSI supercomplex to be a more circular particle with a diameter of about 32 nm. This structure is remarkably similar to that of the IsiA-PS I supercomplex isolated from cyanobacteria grown under iron-deficient condition and to the Pcb-PSI supercomplex isolated from a low-light adapted Prochlorococcus strain SS120. We conclude that in thylakoid membranes of P. hollandica the trimeric PSI complex is surrounded by the ring of 18 Pcb subunits and in so doing significantly increases the size of the light-harvesting system of PSI.

KEY WORDS: Prochlorothrix hollandica, Photosystem, pcbA, ligh-harvesting