Nature 355, 848 - 850 (27 February 1992); doi:10.1038/355848a0

Genetically modified photosynthetic antenna complexes with blueshifted absorbance bands

G. J. S. Fowler, R. W. Visschers^*, G. G. Grief, R. van Grondelle^* & C. N. Hunter

Krebs Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Western Bank, Sheffield, S10 2UH, UK
^*Department of Physics and Astronomy, Free University of Amsterdam, De Boelelaan 1081,1081 HV Amsterdam, The Netherlands

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, Tyr44 and Tyr45, in the a subunit of the peripheral light-harvesting complex of Rhodobacter sphaeroides, a purple photosynthetic bacterium that provides a well characterized system for site-specific mutagenesis^1–3. By constructing single (Tyr44,Tyr45-;PheTyr) and then double (Tyr44, Tyr45PheLeu) 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.

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