2 FUNCTIONALLY DISTINCT FORMS OF THE PHOTOSYSTEM-II REACTION-CENTER PROTEIN D1 IN THE CYANOBACTERIUM SYNECHOCOCCUS SP PCC 7942

The cyanobacterium Synechococcus sp. PCC 7942 possesses a small psbA m ultigene family that codes for two distinct forms of the photosystem I I reaction-center protein D1 (D1:1 and D1:2). We showed previously tha t the normally predominant D1 form (D1:1) was rapidly replaced with th e alternative D1:2 when cells adapted to a photon irradiance of 50 mum ol/m-2.s-1 are shifted to 500 mumol.m-2.s-1 and that this interchange was readily reversible once cells were allowed to recover under the or iginal growth conditions. By using the psbA inactivation mutants R2S2C 3 and R2K1 (which synthesize only D1:1 and D1:2, respectively), we sho wed that this interchange between D1 forms was essential for limiting the degree of photoinhibition as well as enabling a rapid recovery of photosynthesis. In this report, we have extended these findings by exa mining whether any intrinsic functional differences exist between the two D1 forms that may afford increased resistance to photoinhibition. Initial studies on the rate of D1 degradation at three photon-irradian ces (50, 200, and 500 mumol.m-2.s-1) showed that the rates of degradat ion for both D1 forms increase with increasing photon flux density but that there was no significant difference between D1:1 and D1:2. Analy sis of light-response curves for oxygen evolution for the mutants R2S2 C3 and R2K1 revealed that cells with photosystem II reaction centers c ontaining D1:2 have a higher apparent quantum yield (almost-equal-to 2 5%) than cells possessing D1:1. Further studies using chlorophyll a fl uorescence measurements confirmed that R2K1 has a higher photochemical yield than R2S2C3; that is, a more efficient conversion of excitation energy from photon absorption into photochemistry. We believe that th e higher photochemical efficiency of reaction centers containing D1:2 is causally related to the preferential induction of D1:2 at high ligh t and thus may be an integral component of the protection mechanism wi thin Synechococcus sp. PCC 7942 against photoinhibition.