GENETICALLY MODIFIED PHOTOSYNTHETIC ANTENNA COMPLEXES WITH BLUESHIFTED ABSORBENCY BANDS

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 1-3. 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.