G. Friso et al., EVIDENCE FOR CONCURRENT DONOR AND ACCEPTOR SIDE PHOTOINDUCED DEGRADATION OF THE D1-PROTEIN IN ISOLATED REACTION CENTERS OF PHOTOSYSTEM-II, Biochimica et biophysica acta, 1144(3), 1993, pp. 265-270
By varying the period of illumination or by changing the concentration
of the quinone electron acceptor, 2,5-dibromo-3-methyl-6-isopropyl-p-
benzoquinone (DBMIB), it has been possible to demonstrate, using immun
oblottimg, that the photoinduced degradation of the D1-protein follows
two distinct pathways in isolated Photosystem II (PS II) reaction cen
tres. During the initial illumination period, with an intermediate lev
el of DBMIB present (0.2 mM), a 23 kDa fragment containing the N-termi
nus of the D1-protein appears. Further illumination facilitates the ac
cumulation of C-terminal fragments of apparent molecular masses 24 kDa
and 16 kDa. The 24 kDa C-terminal fragment is not observed in the abs
ence of DBMIB or when this quinone is present at high levels (1 mM). I
n the latter condition, the photoinduced degradation of the DI-protein
is significantly protected and only the 23 kDa N-terminal fragment is
produced. The immuno-detected band at 16 kDa is weaker than those of
the higher molecular mass breakdown fragments and its appearance seems
to follow conditions similar, but not identical, to those needed to g
enerate the 24 kDa C-terminal fragment. A closer examination of the 16
kDa band revealed that it consists of both C-terminal and N-terminal
portions of the D1-protein, suggesting it arises from a cleavage about
midway in the protein. We conclude that both donor and acceptor side
photoinhibition can occur in the isolated PS II reaction centre, the b
alance between them being controlled by the extent of the illumination
and the level of DBMIB present. The 23 kDa N-terminal fragment is gen
erated during acceptor side photoinhibition, while the 24 kDa C-termin
al fragment is induced by damage on the donor side.