COMPARISON OF PRIMARY CHARGE SEPARATION IN THE PHOTOSYSTEM-II REACTION-CENTER COMPLEX ISOLATED FROM WILD-TYPE AND D1-130 MUTANTS OF THE CYANOBACTERIUM SYNECHOCYSTIS PCC-6803

We compare primary charge separation in a photosystem II reaction cent er preparation isolated from a wildtype (WT) control strain of the cya nobacterium Synechocystis sp. PCC 6803 and from two site-directed muta nts of Synechocystis in which residue 130 of the D1 polypeptide has be en changed from a glutamine to either a glutamate (mutant D1-Gln130Glu ), as in higher plant sequences, or a leucine residue (mutant D1 Gln13 0Leu). The D1-130 residue is thought to be close to the pheophytin ele ctron acceptor. We show that, when P680 is photoselectively excited, t he primary radical pair state P680(+)Ph(-) is formed with a time const ant of 20-30 ps in the WT and both mutants; this time constant is very similar to that observed in Pisum sativum (a higher plant). We also s how that a change in the residue at position D1-130 causes a shift in the peak of the pheophytin Q(x)-band. Nanosecond and picosecond transi ent absorption measurements indicate that the quantum yield of radical pair formation (phi(RP)), associated with the 20-30-ps component, is affected by the identity of the D1-130 residue. We find that, for the isolated photosystem II reaction center particle, phi(RP higher plant) > phi(RP D1-Gln130Glu mutant) > phi(RP WT) > phi(RP D1-Gln130Leu muta nt). Furthermore, the spectroscopic and quantum yield differences we o bserve between the WT Synechocystis and higher plant photosystem II, s eem to be reversed by mutating the D1-130 ligand so that it is the sam e as in higher plants. This result is consistent with the previously o bserved natural regulation of quantum yield in Synechococcus PS II by particular changes in the D1 polypeptide amino acid sequence (Clark, A . K., Hurry, V, M., Gustafsson, P.. and Oquist, G. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 11985-11989).