The preparation and characterisation of native photoreceptor units from the thylakoids of Rhodopseudomonas viridis

The photosynthetic membranes of Rhodopseudomonas viridis consist of a regular array of structural units. Each unit is composed of a central core (thought to contain the reaction centre complex) surrounded by a subdivided ring of protein (of likely antennae function). These individual units can be dissociated from the membrances using a variety of detergent treatments. The absorption spectrum, used as a criterion of a native state, is retained. All of the seven major polypeptides, the four reaction centre polypeptides (cytochrome, H, M and L chain) as well as the three light‐harvesting polypeptides (B1015‐α, β and ξ) are shown to be present. Electron microscopy of the units shows a similar structure to the units within the membrane. surface‐specific iodination of both membranes and units labels predominantly polypeptides H, B1015‐α, and ξ. M and L are weakly labelled. In addition, B1015‐β is labelled in the isolated units. This, with other evidence, supports an allocation of light‐harvesting polypeptides to the outer ring. Further solubilisation of these units separates the reaction centre (as a native complex containing all four polypeptides) from the light‐harvesting polypeptides. The light‐harvesting polypeptides are obtained in a form containing all three polypeptides and bound pigment, however the peak at 1015 nm corresponding to native bacteriochlorophyll b is lost.