Circadian synthesis of light-harvesting-chlorophyll-proteins in Euglena gracilis is under translational control

Two proteins with apparent molecular masses of 17 and 24 kD that are synthe sized in a circadian manner in the phytoflagellate Euglena gracilis, were r ecognized as proteins belonging to the family of light-harvesting-chlorophy ll-proteins (LHCPs) of class I (17 kD) and of class II (24 kD). Identificat ion was achieved by N-terminal sequencing of the proteins isolated from two -dimensional polyacrylamide gels and by detection with an anti-LHCP II seru m. While it was found that the total amount of LHCPs remains almost constan t, when Euglena is grown under diurnal conditions (12 h light and 12 h dark ), we could show that the amount of newly synthesized 17 and 24 kD proteins varies about 20-fold with a maximum of synthesis in the light phase. In co ntrast, the analysis of the mRNA levels at different times revealed only mi nor differences in the stationary concentration of the LHCP specific mRNA, indicating that the control of LHCP synthesis is at the translational level . Principally, the same finding was obtained using inhibitors of transcript ion. Thus, it is concluded that the expression of LHCPs in Euglena gracilis in contrast to that of higher plants is primarily regulated at the transla tional level.