Regulation by light and ethanol of the synthesis of the light-harvesting chlorophyll a/b-binding protein of photosystem II in Euglena.

In dark-grown Euglena, a single 122-kdalton (kDa) precursor to the light-harvesting chlorophyll a/b-binding protein of photosystem II (pLHCPII) was synthesized at a very low rate and LHCPII synthesis was undetectable as determined by pulse-labeling with [(35)S]sulfate and immunoprecipitation with a specific antibody against Euglena LHCPII. Synthesis of a 207-, 161-, 122- and 110-kDa pLHCPII was detected after light exposure, with the 207-kDa pLHCPII being the most abundant pLHCPII synthesized. The rate of synthesis of all four pLHCPIIs and the 25.6-kDa and 27.2-kDa LHCPIIs increased in the first 12-24 h of light exposure and then declined. The maximal rate of LHCPII synthesis in the light was 50-100-fold greater than the rate in darkness. Addition of ethanol at the time of light exposure inhibited LHCPII synthesis, indicating that induction is catabolite-sensitive. The halflife of pLHCPII in the light or in darkness was 20 min. Therefore, the light induction of LHCPII is the result of an increased rate of synthesis rather than a decreased rate of degradation. Transfer of illuminated cells to darkness resulted in an 80% decrease in the rate of pHLCPII synthesis during the first 0.5 h. Illuminated cells returned to darkness continued to synthesize both 207-kDa pLHCPII and LHCPII for at least 5 h. Light exposure or ethanol addition did not increase the level of translatable RNA for LHCPII. The 50-100-fold catabolite-sensitive increase in the rate of LHCPII synthesis in the absence of a concomitant increase in the level of translatable RNA for LHCPII indicates that in Euglena, the synthesis of LHCPII is controlled at the translational rather than at the transcriptional level.