Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum.

We have investigated the adaptation of the light-harvesting system of the photosynthetic bacterium Phaeospirillum molischianum (DSM120) to very low light conditions. This strain is able to respond to changing light conditions by differentially modulating the expression of a family of puc operons that encode for peripheral light-harvesting complex (LH2) polypeptides. This modulation can result in a complete shift between the production of LH2 complexes absorbing maximally near 850 nm to those absorbing near 820 nm. In contradiction to prevailing wisdom, analysis of the LH2 rings found in the photosynthetic membranes during light adaptation are shown to have intermediate spectral and electrostatic properties. By chemical cross-linking and mass-spectrometry we show that individual LH2 rings and subunits can contain a mixture of polypeptides derived from the different operons. These observations show that polypeptide synthesis and insertion into the membrane are not strongly coupled to LH2 assembly. We show that the light-harvesting complexes resulting from this mixing could be important in maintaining photosynthetic efficiency during adaptation.

Organisation and function of the Phaeospirillum molischianum photosynthetic apparatus.

We have investigated the organisation of the photosynthetic apparatus in Phaeospirillum molischianum, using biochemical fractionation and functional kinetic measurements. We show that only a fraction of the ATP-synthase is present in the membrane regions which contain most of the photosynthetic apparatus and that, despite its complicated stacked structure, the intracytoplasmic membrane delimits a single connected space. We find that the diffusion time required for a quinol released by the reaction centre to reach a cytochrome bc1 complex is about 260 ms. On the other hand, the reduction of the cytochrome c chain by the cytochrome bc1 complex in the presence of a reduced quinone pool occurs with a time constant of about 5 ms. The overall turnover time of the cyclic electron transfer is about 25 ms in vivo under steady-state illumination. The sluggishness of the quinone shuttle appears to be compensated, at least in part, by the size of the quinone pool. Together, our results show that P. molischianum contains a photosynthetic system, with a very different organisation from that found in Rhodobacter sphaeroides, in which quinone/quinol diffusion between the RC and the cytochrome bc1 is likely to be the rate-limiting factor for cyclic electron transfer.