title: Genetically modified photosynthetic antenna complexes with blueshifted absorbance bands.
abstract: Light energy for photosynthesis is collected by the antenna system, creating an excited state which migrates energetically 'downhill'. To achieve efficient migration of energy the antenna is populated with a series of pigments absorbing at progressively redshifted wavelengths. This variety in absorbing species in vivo has been created in a biosynthetically economical fashion by modulating the absorbance behaviour of one kind of (bacterio)chlorophyll molecule. This modulation is poorly understood but has been ascribed to pigment-pigment and pigment-protein interactions. We have examined the relationship between aromatic residues in antenna polypeptides and pigment absorption, by studying the effects of site-directed mutagenesis on a bacterial antenna complex. A clear correlation was observed between the absorbance of bacteriochlorophyll a and the presence of two tyrosine residues, alpha Tyr44 and alpha Tyr45, in the alpha subunit of the peripheral light-harvesting complex of Rhodobacter sphaeroides, a purple photosynthetic bacterium that provides a well characterized system for site-specific mutagenesis. By constructing single (alpha Tyr44, alpha Tyr45----PheTyr) and then double (alpha Tyr44, alpha Tyr45----PheLeu) site-specific mutants, the absorbance of bacteriochlorophyll was blueshifted by 11 and 24 nm at 77 K, respectively. The results suggest that there is a close approach of tyrosine residues to bacteriochlorophyll, and that this proximity may promote redshifts in vivo.
authors.affiliation: Krebs Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, UK.
authors.forename: G J
authors.initials: GJ
authors.lastname: Fowler
authors.forename: R W
authors.initials: RW
authors.lastname: Visschers
authors.forename: G G
authors.initials: GG
authors.lastname: Grief
authors.forename: R
authors.initials: R
authors.lastname: van Grondelle
authors.forename: C N
authors.initials: CN
authors.lastname: Hunter
mesh_terms: D000595:Amino Acid Sequence; D002942:Circular Dichroism; D045342:Light-Harvesting Protein Complexes; D046911:Macromolecular Substances; D008961:Models, Structural; D008969:Molecular Sequence Data; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000378:metabolism*; D012241:Rhodopseudomonas / Q000235:genetics / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
chemical_list: D045342:Light-Harvesting Protein Complexes; D046911:Macromolecular Substances; D045322:Photosynthetic Reaction Center Complex Proteins