E. Bracht et A. Trebst, HYPOTHESIS ON THE CONTROL OF D1 PROTEIN-TURNOVER BY NUCLEAR CODED PROTEINS IN CHLAMYDOMONAS-REINHARDTII, Zeitschrift fur Naturforschung. C, A journal of biosciences, 49(7-8), 1994, pp. 439-446
A hypothesis is presented on the events in the degradation of the D1 p
rotein of photosystem II in the light. It proposes the existence of a
nuclear encoded cleavage system that is turning over and which is modu
lated by its phosphorylation state. A new experimental approach is pre
sented in which the D1 protein degradation under photoinhibitory light
is tested in Chlamydomonas reinhardtii grown under phosphate deficien
cy and pretreated with cycloheximide. Under these conditions the degra
dation of the D1 protein is delayed whereas in Chlamydomonas reinhardt
ii grown in full medium the D1 protein is rapidly disappearing in high
light upon addition of chloramphenicol (CAP) or lincomycin for inhibi
ting the resynthesis of the D1 protein. Cycloheximide (CHI) has little
effect on photoinhibition in such control cells. In cells grown, howe
ver, for 20 h in phosphate deficiency - such that there is not yet los
s of photosynthesis capacity - pretreatment with cycloheximide or cana
vanine in low light the degradation of the D1 protein even in 6 h high
light is prevented to an appreciable extent. Further addition of CAP
or lincomycin has only a small effect. [C-14] leucine incorporation wa
s used to show that there is no resynthesis and that the presence of t
he D1. protein is due to a delay of degradation. The results are inter
preted to show that excess high light which converts the D1 protein in
to a potentially, degradable mode is not sufficient for degradation of
the D1 protein. A cleavage system is needed as well. It is postulated
that under phosphate deficiency and pretreatment with CHI or canavani
ne a nuclear coded cleavage system for the D1 protein is depleted, i.e
. the cleavage system for the rapidly turning over D1 is also turning
over. It is shown that under phosphate deficiency an alkaline phosphat
ase activity in the chloroplast and the thylakoid membrane of Chlamydo
monas reinhardtii is increased. It is proposed that the ratio of kinas
e/phosphatase converts an active, stable phosphorylated cleavage syste
m into a labile unphosphorylated and turning over state.