Rhodobacter sphaeroides is a purple bacterium whose photosynthetic apparatus is composed of cytochromes, an ATPase, a reaction centre (RC), and two light harvesting antenna complexes (LH1 and LH2) that funnel excitation energy towards the RC. The photosynthetic unit formed from these different components is located within the highly invaginated intracytoplasmic membrane (ICM) (Figure 1.1). The RC is the primary photochemical site made up of the three subunits L, H, and M, and is the site at which charge separation is initiated. LH1 and LH2 form ring-like structures and are both heterodimers composed of two hydrophobic polypeptides called a and b. These form transmembrane helices containing 50–60 amino acids and determine the position, orientation and environment of light harvesting pigments such as bacteriochlorophyll and carotenoids (Figure 1.2 shows a model of the LH1 complex).
LH2 is the peripheral light harvesting complex present in variable amounts according to the incident light intensity, whilst LH1 is the core light harvesting complex present in a fixed stoichiometry to the RC. Studies on purple bacteria, e.g., Rhodopseudomonas palustris have shown LH1 forms a large cyclic structure with a ‘hole’ in the middle that allows the complex to encircle the RC (Figure 1.3) (Roszak et al., 2003). It has been questioned whether LH1 forms a complete ring in the presence of RC as this would affect the flow of electrons from the RC to cytochrome bc1.
Together, the RC and LH1 form core complexes in several species of purple bacteria, and in Rb. sphaeroides the RC-LH1 core complexes are found as dimers. This has been demonstrated by tapping mode atomic force microscopy (AFM) carried out on native photosynthetic membranes (Bahatyrova et al., 2004) (Figure 1.4).
A number of factors have been found to affect dimerisation of these core complexes. One such factor is the protein PufX which has been shown to be essential for photosynthetic growth and promote the formation of core complex dimers. Only monomer core complexes are isolated in the absence of PufX (Siebert et al., 2004). Like the LH1 complex, PufX is also present in a 1:1 stoichiometry with the RC……