The antenna complexes of the purple non-sulfur photosynthetic bacterium Rhodocyclus tenuis. Structural and spectral characterization

Abstract

The photoreceptor complex (B885-RC) and the peripheral antenna complex (B800-860) were isolated from photosynthetic membranes of the purple non-sulfur bacterium Rhodocyclus tenuis DSM 109 using a detergent combination of Deriphate-160 and octyl glucoside and subsequent linear sucrose gradient centrifugation. The two complexes were characterized by room-temperature absorption, circular dichroism and fluorescence spectroscopy. The B800-860 complex has a more red-shifted B860 absorbance band. The alpha,beta-polypeptides were purified with a reverse-phase HPLC system and resolved at a ratio of 1:1 in the B800-860 complex and at an overall ratio of 1:1 for the B885-RC complex. The complete amino acid sequences of the alpha and beta polypeptides of the B800-860 and B885-RC complexes were determined by micro-sequencing analysis and mass spectrometry. The B800-860-alpha polypeptide possesses an identical N-terminal domain (the first 15 residues) to Rhodobacter sphaeroides B800-850-alpha polypeptide. The central hydrophobic and C-terminal domains of the B800-860-alpha,beta polypeptides show a number of B870/880-like structural elements in which, of special interest, is the WWSEF cluster in the C-terminal domain of the B800-860-alpha polypeptide which is very similar to the WWEF cluster in the same region of Rhodopseudomonas viridis B1015-alpha polypeptide. The more red-shifted absorption characteristic of the 860-nm bacteriochlorophylls could most probably be related to the B870/880-like polypeptide features in the central hydrophobic domains and the C-terminal domains of the B800-860-alpha,beta polypeptides. The hydropathy plot of the B800-860-alpha polypeptide exhibits an extended C-terminal hydrophobic segment indicative of a second membrane-contacting domain, which has not been found in the antenna polypeptides of the purple bacteria with intracytoplasmic membranes. Further sequence analysis revealed the existence of multiple forms of the B885-alpha,beta polypeptides, the B885-alpha 1,alpha 2 polypeptides, and the B885-beta 1,beta 2 polypeptides. The B885-Alpha 2 polypeptide shows an identical sequence to the B885-alpha 1 polypeptide, but it is 12 amino acid residues shorter than the B885-alpha 1 polypeptide at the C-terminal. The two species of the B885-beta polypeptides were identified as an identical sequence with only one amino acid residue variation at sequence position 34, where the B885-beta 1 has a valine residue and the B885-beta 2 polypeptide an isoleucine residue. The possible correlation between the intensity of the near-infrared circular dichroic signal and the specific structural features of the alpha and beta core antenna polypeptides is also discussed.