Antenna mixing in photosynthetic membranes from Phaeospirillum molischianum

Abstract

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.chromatic adaptation | membrane protein | photosynthetic bacteria | Rhodospirillum molischianum P hotosynthetic bacteria are able to efficiently convert incident light into chemical potential energy via a light-driven cyclic electron transfer pathway (1). This process depends on the efficient absorption of incident light energy and the transfer of this energy to the reaction center. Measurements of the quantum efficiency of the light-harvesting apparatus indicate that very little energy is lost during the migration of the exciton from the point of initial absorption to the reaction center (1). The light-harvesting system of many bacteria contains two different pigment-protein complexes. The core complex contains one or two reaction centers in close association with a light-harvesting system (LH1) and a smaller peripheral complex (LH2) that transfers the absorbed excitation energy to the core complex. Both types of light-harvesting complex, LH1 and LH2, are constructed from oligomers of a similar basic subunit containing 2 polypeptides (α and β) with associated bacteriochlorophyll and carotenoid pigments. Their structures are known from x-ray crystallography (2-4), electron crystallography (5), and more recently from atomic force microscopy (6-11).The LH2 complexes are circular oligomers of typically 9 αβ subunits (3), in which 18 long wavelength-absorbing bacteriochlorophyll molecules are sandwiched between the inner ring of α-polypeptides and an outer-ring of β-polypeptides. A second series of 9 bacteriochlorohpyll molecules, closer to the cytoplasmic membrane surface, occupies gaps between the β-polypeptides. This general architecture is slightly variable, with in particular the number of monomeric units forming the circular architecture depending on the species and environmental conditions (2,7,12,13). In the species studied in this work, Phaeospirillum (Ph.) molischianum...

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