SPECTRAL ALTERATIONS IN RHODOBACTER-CAPSULATUS MUTANTS WITH SITE-DIRECTED CHANGES IN THE BACTERIOCHLOROPHYLL-BINDING SITE OF THE B880 LIGHT-HARVESTING COMPLEX

Site-directed mutagenesis has suggested that conserved histidine and a lanine residues in the alpha-subunit of the B880 (LHI) antenna complex of Rhodobacter capsulatus (alpha His32 and alpha Ala28) form part of the bacteriochlorophyll binding site (Bylina, E.J., Robles, S.J. and Y ouvan, D.C. (1988) Isr. J. Chem. 28, 73-78). Spectroscopic characteriz ation of chromatophores from alpha Ala28 mutants at 77 K revealed: (i) red shifts in B880 absorption and emission maxima of similar to 6 and 10 nm, respectively, with a serine exchange; (ii) red shifts of 3 nm with a glycine exchange; (iii) and no significant shifts with a cystei ne exchange, despite a reduction of similar to 50% in B880 level. The strains with the serine and glycine exchanges showed characteristic fl uorescence polarization increases over the red-edge of the B880 band, suggesting that the absorption red shifts arose from altered pigment-p rotein interactions rather than from increased oligomerization states that would be expected to show markedly diminished and red shifted ris es in polarization (Westerhuis, W.H.J., Farchaus, J.W. and Niederman, R.A. (1993) Photochem. Photobiol. 58, 460-463). Excitation spectra of strains with alpha His32 to glutamine and alpha Ala28 to histidine exc hanges, thought to be depleted in B880, revealed low levels of a 'pseu do-B880' complex with blue-shifted maxima and fluorescence polarizatio n rises; when excited directly into this component, the former strain showed an emission spectrum similar to that of B880. An essentially wi ld-type electrochromic carotenoid response was observed only in the B8 80-containing mutants, since membranes isolated from the B880-depleted strains exhibited an increased permeability to ions.