REDOX REGULATION OF LIGHT-HARVESTING COMPLEX-II AND CAB MESSENGER-RNAABUNDANCE IN DUNALIELLA-SALINA

We demonstrate that photosynthetic adjustment at the level of the ligh t-harvesting complex associated with photosystem II (LCH II) in Dunali ella salina is a response to changes in the redox state of intersystem electron transport as estimated by photosystem II (PSII) excitation p ressure. To elucidate the molecular basis of this phenomenon, LHCII ap oprotein accumulation and cab mRNA abundance were examined. Growth reg imes that induced low, but equivalent, excitation pressures (either 13 degrees C/20 mu mol m(-2) s(-1) or 30 degrees C/150 mu mol m(-2) s(-1 )) resulted in increased LHCII apoprotein and cab mRNA accumulation re lative to algal cultures grown under high excitation pressures (either 13 degrees C/150 mu mol m(-2) s(-1) or 30 degrees C/2500 mu mol m(-2) s(-1)). Thermodynamic relaxation of high excitation pressures, accomp lished by shifting cultures from a 13 to a 30 degrees C growth regime at constant irradiance for 12 h, resulted in a 6- and 8-fold increase in LHCII apoprotein and cab mRNA abundance, respectively. Similarly, p hotodynamic relaxation of high excitation pressure, accomplished by a shift from a light to a dark growth regime at constant temperature, re sulted in a 2.4- to 4-fold increase in LHCII apoprotein and cab mRNA l evels, respectively. We conclude that photosynthetic adjustment to tem perature mimics adjustment to high irradiance through a common redox s ensing/signaling mechanism. Both temperature and light modulate the re dox state of the first, stable quinone electron acceptor of PSII, whic h reflects the redox poise of intersystem electron transport. Changes in redox poise signal the nucleus to regulate cab mRNA abundance, whic h, in turn, determines the accumulation of light-harvesting apoprotein . This redox mechanism may represent a general acclimation mechanism f or photosynthetic adjustment to environmental stimuli.