CHANGES OF AMINO-ACID-SEQUENCE IN PEST-LIKE AREA AND QEEET MOTIF AFFECT DEGRADATION RATE OF D1 POLYPEPTIDE IN PHOTOSYSTEM-II

The degradation rate of the D1 polypeptide was measured in three Synec hocystis PCC 6803 mutants in vivo. Mutations were introduced into a pu tative cleavage area of the D1 polypeptide (QEEET motif) and into the PEST-like area. PEST sequences are often found in proteins with a high turnover rate. The QEEET-motif mutants are CA1 [Delta(E242-E244);Q241 H] and E243K, and the third mutation, E229D, was directed to the PEST- like area. During high-light illumination (1500 mu mol photons m(-2) s (-1))that induced photoinhibition of photosystem II (PSII), the half-l ife time of the D1 polypeptide in mutant E229D (t(1/2) = 35 min) was a bout twice as long as in AR (control strain) cells (t(1/2) = 19 min). In growth light (40 mu mol photons m(-2) s(-1)), the degradation rate of the D1 polypeptide in E229D and AR strains was the same (t(1/2) app roximate to 5 h). In growth light the D1 polypeptide was degraded fast er in both QEEET-motif mutants than in the AR strain, but in photoinhi bitory light the degradation rates were similar. According to these re sults, the highly conservative QEEET motif as such is not required for the proteolytic cut of the D1 polypeptide, but it does affect the rat e of degradation. No simple correlation existed between the degradatio n rate of the D1 polypeptide and the susceptibility of PSII to photoin hibition in mutant and AR cells under our experimental conditions.