CHLOROPHYLL-A AVAILABILITY AFFECTS PSBA TRANSLATION AND D1 PRECURSOR PROCESSING IN-VIVO IN SYNECHOCYSTIS SP. PCC-6803

Transcript accumulation and translation of psbA as well as processing of the D1 precursor protein were investigated in relation to chlorophy ll availability in vivo in cyanobacterial strains lacking photosystem I (PS I). The psbA transcript level was almost independent of chloroph yll availability and was approximate to 3-fold Bower in darkness than in continuous light (5 mu E m(-2) s(-1)). Upon illumination, it reache d a steady-state level within several hours, Upon growth under light-a ctivated heterotrophic growth conditions (LAHG) in the EPS I-less stra in, D1 synthesis occurred immediately upon illumination, However, in F S I-less/chlL(-) cells, which lacked the light-independent chlorophyll biosynthesis pathway and had very low chlorophyll levels after LAHG g rowth, very little D1 synthesis occurred upon illumination, and the sy nthesis rate increased with time. This result suggests a translational control of D1 biosynthesis related to chlorophyll availability, Upon illumination, initially a high Bevel of the nonprocessed D1 precursor was observed by pulse labeling and immunodetection in LAHG-grown PS I- less/chlL(-) cells but not in PS I-less cells. A significant amount of the D1 precursor eventually was processed to mature DB, and the half- life of the D1 precursor decreased as the chlorophyll content of the c ells increased. The Ell processing enzyme CtpA was found to be present at similar levels regardless illumination or chlorophyll levels. We c onclude that, directly or indirectly. chlorophyll availability is need ed for HPI translation as well as for efficient professing of the D1 p recursor.