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In this paper, we have examined, using FT resonance Raman spectroscopy, the bacteriochlorophyll (BChl) binding sites in the peripheral light-harvesting complexes extracted from a number of purple bacterial strains. A comparison of interactions of the BChl molecules with their binding sites in these LH2 complexes, together with the primary sequences of the alpha and beta polypeptides, allows three amino acids to be proposed to be involved in the hydrogen bonding of the 9-keto carbonyl of one of the 850-nm-absorbing pair of BChl molecules. Specifically, we show that one keto carbonyl group, which is strongly hydrogen bonded in Rhodobacter sphaeroides LH2, is involved in much weaker interactions in the LH2 complexes from all the other species studied (i.e., Rhodobacter capsulatus, Rubrivivax gelatinosus, Rhodopseudomonas palustris, Rhodopseudomonas acidophila, and Rhodopseudomonas cryptolactis). This is correlated with the presence of three polar amino acids in the primary sequence of the alpha polypeptide of Rb. sphaeroides which are absent in the sequences from all the other bacteria and probably close to a chromophore. These three residues are a serine at position -4, a threonine at position +6 and another serine at position +17 (numbering relative to the conserved histidine, considered as position 0), in the alpha polypeptide of Rb. sphaeroides. Furthermore, the study of the interactions in natural B800-820 complexes shows that the two 2-acetyl groups of the 820-nm-absorbing BChl molecules are free from hydrogen-bonding interactions. In the light of previous site-selected mutagenesis studies, the lack of such hydrogen bonds seems to be a general phenomenon, associated with the 820-nm absorption of LH2 complexes, and suggests that hydrogen-bonding interactions have a precise molecular role in finely tuning the functional properties of these complexes.

This study describes the characterization of orf358, an open reading frame of previously unidentified function, in the purple bacterium Rubrivivax gelatinosus. A strain in which orf358 was disrupted exhibited a phenotype similar to the wild type under photosynthesis or low-aeration respiratory growth conditions. In contrast, under highly aerated respiratory growth conditions, the wild type still produced bacteriochlorophyll a (Bchl a), while the disrupted strain accumulated a compound that had the same absorption and fluorescence emission spectra as Mg-protoporphyrin but was less polar, suggesting that it was Mg-protoporphyrin monomethylester (MgPMe). These data indicated a blockage in Bchl a synthesis at the oxidative cyclization stage and implied the coexistence of two different mechanisms for MgPMe cyclization in R. gelatinosus, an anaerobic mechanism active under photosynthesis or low oxygenation and an aerobic mechanism active under highoxygenation growth conditions. Based on these results as well as on sequence analysis indicating the presence of conserved putative binuclear-iron-cluster binding motifs, the designation of orf358 as acsF (for aerobic cyclization system Fe-containing subunit) is proposed. Several homologs of AcsF were found in a wide range of photosynthetic organisms, including Chlamydonomas reinhardtii Crd1 and Pharbitis nil PNZIP, suggesting that this aerobic oxidative cyclization mechanism is conserved from bacteria to plants.Purple bacteria perform anoxygenic photosynthesis on the basis of a bacteriochlorophyll-mediated process. Their photosynthetic apparatus, related to plant photosystem II, comprises three pigment-protein complexes: the light-harvesting antennae LHI and LHII and the reaction center, associated with bacteriochlorophyll a (Bchl a) and carotenoids (13,15,23).