The effect of different levels of the B800-850 light-harvesting complex on intracytoplasmic membrane development in Rhodobacter sphaeroides

Abstract

A role for the peripheral (B800-850) light-harvesting complex in vesicularization of the Rhodobacter sphaeroides intracytoplasmic membrane (ICM), suggested from studies in mutant strains lacking one or more of the pigment-protein complexes, was examined further in the wild-type strain NCIB 8253 grown at high ( approximately 1000 W m-2), moderate ( approximately 300 W m-2), and low ( approximately 100 W m-2) light intensities. The resulting ICM vesicles (chromatophores) had B800-850 levels related inversely to irradiance and banded in rate-zone sedimentation at approximately 1.10, 1.09, and 1.07 g ml-1, respectively. Equilibrium centrifugation on iso-osmotic gradients indicated that this distinct sedimentation behavior resulted solely from differences in hydrodynamic radii. These size differences were confirmed by gel-exclusion chromatography and in electron micrographs of thin-sectioned cells. A pulse-chase study of ICM growth following a tenfold reduction in light intensity showed a relatively slow equilibration of membrane proteins during adaptation, and that new protein was incorporated largely into additional ICM formed at the lowered illumination level, giving rise to chromatophores of reduced size and elevated B800-850 content. These results provide further evidence for a model in which the B800-850 complex both drives development of vesicular ICM in Rba. sphaeroides and determines the size of resulting vesicles.