Energy migration in Rhodobacter sphaeroides mutants altered by mutagenesis of the peripheral LH2 complex or by removal of the core LH1 complex

AbstractThe photosynthetic apparatus of the purple bacterium Rhodobacter sphaeroides is organised so that light energy absorbed by the peripheral antenna (LH2) complexes migrates towards the core (LH1) complex, before being trapped by the reaction centre (RC). This migration and trapping process has been studied in mutants where the energy levels of the LH2 BChls have been raised by mutagenesis of the C-terminal aromatic residues (Fowler, G.J.S., Visschers, R.W., Grief, G.G., Van Grondelle, R. and Hunter, C.N. (1992) Nature 355, 848–850), and in a mutant which lacks the core complex. In the former case, the alterations to the LH2 complexes did not prevent efficient energy transfer to the LHI-RC complex, but fluorescence emission spectra indicated that the equilibrium of energy within the system was affected so that back transfer from the LH1-RC core is minimised. This mimics the situation found in some other bacteria such as Rhodopseudomonas acidophila and Rps. cryptolactis. In the mutant lacking LH1, energy is transferred from LH2 directly to the RC, despite the absence of the core antenna. Energy transfer efficiencies for carotenoids and LH2 to LH1 were measured for the blue-shifted LH2 mutants, and were found to be high (70%) in each case. These data, together with measurements of excitation annihilation as a function of incident excitation energy, were used to estimate the domain sizes for energy transfer in these mutants. In the LH2 mutants, domains of about 50 to 170 core BChls were found, depending on the type of mutation. One effect of the removal of LH1 appears to be the reorganisation of the peripheral LH2 antenna to form domains of at least 250 BChls