Purification and characterisation of photosystem I from the unusual cyanobacterium Gloeobacter violaceus

Abstract

Photosystem I from the highly unusual primitive cyanobacterium Gloeobacter violaceus was purified. The isolated Photosystem I complexes have the same subunit composition as complexes from other cyanobacteria as determined by high resolution gel electrophoresis and immunoblotting. They exist both in a trimeric and in a monomeric form as shown by gel filtration and electron microscopy. Image analysis of single particles shows that the size and shape of the particles closely resembles other cyanobacterial Photosystem I particles. Flash induced absorption difference spectra show a classical P700 shape with kinetics and relaxation times similar to Photosystem I from Synechocystis, but its amplitude indicates a much bigger antenna size, resembling the spectra of higher plants rather than those of other cyanobacteria. 77 K fluorescence emission spectra lack the typical long wavelength emission peaks of cyanobacterial Photosystem I. In addition, the preparation contains novel polypeptides not related to known Photosystem I subunits. Immunoblotting shows the possible existence of a membrane intrinsic light-harvesting protein related to LHC proteins of Chromophytes. Moreover, electron microscopy reveals a novel T-shaped complex that has not been observed so far.