ENERGY MIGRATION IN RHODOBACTER-SPHAEROIDES MUTANTS ALTERED BY MUTAGENESIS OF THE PERIPHERAL LH2 COMPLEX OR BY REMOVAL OF THE CORE LH1 COMPLEX

The photosynthetic apparatus of the purple bacterium Rhodobacter sphae roides is organised so that light energy absorbed by the peripheral an tenna (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 LH 2 BChls have been raised by mutagenesis of the C-terminal aromatic res idues (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 l acks the core complex. In the former case, the alterations to the LH2 complexes did not prevent efficient energy transfer to the LH1-RC comp lex, but fluorescence emission spectra indicated that the equilibrium of energy within the system was affected so that back transfer from th e LH1-RC core is minimised. This mimics the situation found in some ot her bacteria such as Rhodopseudomonas acidophila and Rps. cryptolactis . In the mutant lacking LH1, energy is transferred from LH2 directly t o the RC, despite the absence of the core antenna. Energy transfer eff iciencies for carotenoids and LH2 to LH1 were measured for the blue-sh ifted LH2 mutants, and were found to be high (70%) in each case. These data, together with measurements of excitation annihilation as a func tion of incident excitation energy, were used to estimate the domain s izes 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 mu tation. One effect of the removal of LH1 appears to be the reorganisat ion of the peripheral LH2 antenna to form domains of at least 250 BChl s.