Characterisation of the photosynthetic complexes from the marine gammaproteobacterium Congregibacter litoralis KT71

Highlights

• Biophysical and structural characterisation of the photosynthetic complexes in Congregibacter litoralis.

• The RC-LH1 complex consists of a reaction centre surrounded by 16-mer antenna ring with no gap or pore.

• The cells produce a peripheral antenna LH2 but the LH2/RC-LH1 ratio is substantially smaller than in purple bacteria.

• Based on primary sequence and phylogenetic analyses LH2 is predicted to be a nonameric complex.

• The RC-LH1 is with gammaproteobacterial complexes but LH2 has some alphaproteobacterial complex features.

Abstract

Possibly the most abundant group of anoxygenic phototrophs are marine photoheterotrophic Gammaproteobacteria belonging to the NOR5/OM60 clade. As little is known about their photosynthetic apparatus, the photosynthetic complexes from the marine phototrophic bacterium Congregibacter litoralis KT71 were purified and spectroscopically characterised. The intra-cytoplasmic membranes contain a smaller amount of photosynthetic complexes when compared with anaerobic purple bacteria. Moreover, the intra-cytoplasmic membranes contain only a minimum amount of peripheral LH2 complexes. The complexes are populated by bacteriochlorophyll a, spirilloxanthin and two novel ketocarotenoids, with biophysical and biochemical properties similar to previously characterised complexes from purple bacteria. The organization of the RC-LH1 complex has been further characterised using cryo-electron microscopy. The overall organisation is similar to the complex from the gammaproteobacterium Thermochromatium tepidum, with the type-II reaction centre surrounded by a slightly elliptical LH1 antenna ring composed of 16 αβ-subunits with no discernible gap or pore. The RC-LH1 and LH2 apoproteins are phylogenetically related to other halophilic species but LH2 also to some alphaproteobacterial species. It seems that the reduction of light-harvesting apparatus and acquisition of novel ketocarotenoids in Congregibacter litoralis KT71 represent specific adaptations for operating the anoxygenic photosynthesis under aerobic conditions at sea.