EXPRESSION OF ELIPS AND PS II-S PROTEIN IN SPINACH DURING ACCLIMATIVEREDUCTION OF THE PHOTOSYSTEM-II ANTENNA IN RESPONSE TO INCREASED TIGHT INTENSITIES

The PS II-S protein and the so-called early light-inducible proteins ( ELTPs) are homologous to the chlorophyll alb-binding (Cab) gene produc ts functioning in light-harvesting. The functional significance of the se two CAB homologues is not known although they have been considered to bind pigments and in the case of the PS II-S protein this has been experimentally supported. The role of these two proteins does not appe ar to be light-harvesting but instead they are suggested to play a rol e as quenchers of free chlorophyll molecules during biogenesis and/or degradation of pigment-binding proteins. Such a role would be essentia l to eliminate the toxic and damaging effects that can be induced by f ree chlorophyll in the light. To this end the expression and character istics of the ELIPs and the PS II-S protein were investigated in spina ch leaves acclimating from low to high light intensities. Under these conditions there is a reduction in the antenna size of Photosystem II due to proteolytic digestion of its major chlorophyll alb-binding prot ein (LHC II). During this acclimative proteolysis, up to one third of LHC II can be degraded and consequently substantial amounts of chlorop hyll molecules will lose their binding sites. Our results reveal that there is a close correlation between ELIP accumulation and the onset o f the LHC II degradation as low light-grown spinach leaves are subject ed to increased light intensities. In contrast, there was no change in the relative level of the PS II-S protein during the acclimation proc ess. It is concluded that the role for the ELIPs may be related to bin ding of liberated chlorophyll molecules and quenching of the toxic eff ects during LHC II degradation. In addition it was shown that in spina ch four different ELIP species can be expressed and that they show dif ferent accumulation patterns in response to increased light intensitie s.