The Structure and Function of Some Unusual Variable Antenna Complexes

Abstract

In purple photosynthetic bacteria the extent of the development of the photosynthetic apparatus is regulated by environmental conditions, such as the light-intensity at which the cells are grown.^1,2 In the two most commonly studied species, Rhodobacter sphaeroides and Rhodobacter capsulatus the effect of growing cells at different light-intensities has been well documented.^1–3 The major response to lowering the light-intensity is the increased synthesis of the variable B800-850-complexes (the LH2 complexes). There are however a number of less well-studied species that not only regulate the size of their photosynthetic units in response to changes in the ambient environment, but also have the ability to alter the type of LH2 complex which is synthesised.^4–6 Chromatium vinosum, Rhodopseudomonas palustris and Rhodopseudomonas acidophila are examples of species which show this type of response. The ability to synthesise these extra kinds of antenna complexes appears to correlate with the capacity of these species to grow at very low light-intensities. It is interesting that these different types of LH2 complex have unusual absorption spectra.