Single amino acid substitutions in the B870 α and β light‐harvesting polypeptides of Rhodobacter capsulatus Structural and spectral effects

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Abstract

To obtain information on the structural and functional role of highly conserved amino acid residues in the B870 α and β light‐harvesting polypeptides of Rhodobacter capsulatus, site‐directed mutagenesis was performed. 18 mutants with single amino acid substitutions at nine different positions in the B870 antenna polypeptides were prepared in a B800–850‐lacking strain. The characterization of the resulting phenotypes was based on a quantification of the core‐complex elements (reaction center, light‐harvesting polypeptides, bacteriochlorophyll a and carotenoid) and the core‐complex spectral characteristics (absorption maximum, absorption coefficient and fluorescence intensity). These data generally showed that strong structural effects were caused by the amino acid substitutions. Thus, the three tryptophan exchanges at the position α8 resulted in either the absence of a core complex (αTrp8→Leu), the absence of the core antenna (αTrp8→Ala) or a reduction in the carotenoid content (αTrp8→Tyr). Likewise, the mutants αPro13Gly (i.e. αPro13→Gly), βGly10Val and αPhe23Ala demonstrated an abnormal protein/pigment ratio in the core antenna, while a drastically reduced antenna size resulted from the amino acid exchange βArg45Asp. In contrast to the structural effects, the absorption maxima and the fluorescence intensities of the mutant antennae differed only slightly from the wild type. The strongest blue shift of the bacteriochlorophyll a (8 – 11 nm) was induced by substitutions of the Trp at position α43 (αTrp43→Ala, Leu or Tyr). Contrary to the other spectral effects, the absorption coefficient of bacteriochlorophyll a was strongly influenced by the amino acid substitutions and varied by 1.6‐times less (βArg45Asp) and 1.3‐times greater (αPhe25Ala) than normal. The antenna‐free mutant, αTrp8Ala, yielded a high rate of B800‐850 revertants during phototrophic growth, indicating a direct energy transfer from the B800–850 antenna to the reaction center in these strains. Although conditions for growth were generally observed to influence phenotypic expression, the structural as well as spectral effects were demonstrated to differ to the greatest extent between chemotrophically grown and phototrophically grown cells. Copyright © 1991, Wiley Blackwell. All rights reserved

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BABST, M., ALBRECHT, H., WEGMANN, I., BRUNISHOLZ, R., & ZUBER, H. (1991). Single amino acid substitutions in the B870 α and β light‐harvesting polypeptides of Rhodobacter capsulatus Structural and spectral effects. European Journal of Biochemistry, 202(2), 277–284. https://doi.org/10.1111/j.1432-1033.1991.tb16373.x

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