The 7.5-Å Electron Density and Spectroscopic Properties of a NovelLow-Light B800 LH2 from Rhodopseudomonas palustris

Abstract

A novel low-light (LL) adapted light-harvesting complex II has been isolated from Rhodopseudomonas palustris. Previous work has identified a LL B800–850 complex with a heterogeneous peptide composition and reduced absorption at 850 nm. The work presented here shows the 850 nm absorption to be contamination from a high-light B800–850 complex and that the true LL light-harvesting complex II is a novel B800 complex composed of eight αβ d peptide pairs that exhibits unique absorption and circular dichroism near infrared spectra. Biochemical analysis shows there to be four bacteriochlorophyll molecules per αβ peptide rather than the usual three. The electron density of the complex at 7.5 Å resolution shows it to be an octamer with exact 8-fold rotational symmetry. A number of bacteriochlorophyll geometries have been investigated by simulation of the circular dichroism and absorption spectra and compared, for consistency, with the electron density. Modeling of the spectra suggests that the B850 bacteriochlorophylls may be arranged in a radial direction rather than the usual tangential arrangement found in B800–850 complexes.