STRUCTURAL STUDIES OF WILD-TYPE AND MUTANT REACTION CENTERS FROM AN ANTENNA-DEFICIENT STRAIN OF RHODOBACTER-SPHAEROIDES - MONITORING THE OPTICAL-PROPERTIES OF THE COMPLEX FROM BACTERIAL-CELL TO CRYSTAL

Reaction centers have been crystallized from the antenna-deficient RCO 2 strain of Rhodobacter sphaeroides, and a structural model has been c onstructed at 2.6 Angstrom resolution. The antenna-deficient strain al lows assessment of the structural integrity of the reaction center at each stage in the purification-crystallization procedure. Spectroscopi c evidence indicates that the properties of the reaction center bacter iopheophytins and the primary donor bacteriochlorophylls are modified somewhat on removal of the protein complex from the membrane and that these changes are carried through to the crystal form of the reaction center. The structure of a FM197R/YM177F mutant reaction center has al so been determined to 2.55 Angstrom resolution. The mutant complex sho ws an unexpected change in structure, with a significant reorientation of the new arginine, the incorporation of a new water molecule into t he structure, and rotation of the 2-acetyl carbonyl group of one of th e primary donor bacteriochlorophylls to a more out-of-plane geometry. Changes in the optical spectrum of the FM197R/YMl77F reaction center a re discussed with respect to the altered structure of the complex.