SITE-DIRECTED MUTAGENESIS OF THE LH2 LIGHT-HARVESTING COMPLEX OF RHODOBACTER-SPHAEROIDES - CHANGING BETA-LYS23 TO GLN RESULTS IN A SHIFT IN THE 850 NM ABSORPTION PEAK

The present study describes the construction of a Rhodobacter sphaeroides light-harvesting (LH2) mutant in which the charged residue betaLys23 is changed by site-directed mutagenesis to a Gln residue, and the characterization of the resulting mutant complex by a range of spectroscopic techniques. In the 77 K absorption spectrum of the mutant, the peak equivalent to the 850 nm peak in the wild-type membrane is blue-shifted by approximately 18 nm to 837 nm; except for this blue-shift, the 77 K fluorescence excitation and emission spectra and the circular dichroism spectrum of the mutant are very similar to the equivalent spectra from the wild-type membranes, suggesting that the mutation betaLys23 --> Gln probably does not cause any major changes in the conformation or aggregation state of these membranes. Possible causes of the 18 nm blue-shift in the absorption spectrum are discussed.