Site-directed modification of the ligands to the bacteriochlorophylls of the light-harvesting LH1 and LH2 complexes of Rhodobacter sphaeroides.

The core light-harvesting LH1 complex of Rhodobactersphaeroides consists of an assembly of membrane-spanning alpha and beta polypeptides, each of which binds one bacteriochlorophyll molecule. In this study we have used site-directed mutagenesis to demonstrate that the B880 bacteriochlorophyll binding site of LH1 shows a high degree of specificity for the residue that provides the ligand to the Bchl Mg2+ ion. alpha His0 (alphaH0) was changed to Asn, Leu, and Tyr, and beta His0 (betaH0) to Asn, Gln, Leu, and Tyr; the mutated genes were expressed to yield both LH1-only and LH1 + RC strains, in two different carotenoid backgrounds. None of the alphaH0 mutations formed an LH1 complex either in isolation or with RCs. Of the mutations of betaH0 those to Asn and Gln formed LH1 complexes but in the latter case the complex was very unstable and occurred at very low cellular levels. In a similar study, the alphaH0 and betaH0 residues of the LH2 complex were changed to Asn. However, no complex was formed in either case. FT Raman spectroscopy of the betaH0N mutant LH1 shows perturbation of the interaction state of the keto carbonyl of one Bchl which sheds light on the possible H-bond partners for these keto oxygens. These data directly address the B880 binding site of the core LH1 complex and show that it is subtly different from the B850 binding site of the peripheral LH2 complex. A model of this binding site may be proposed from these results.