COMPARISON OF PSBO AND PSBH DELETION MUTANTS OF SYNECHOCYSTIS PCC-6803 INDICATES THAT DEGRADATION OF D1 PROTEIN IS REGULATED BY THE Q(B) SITE AND DEPENDENT ON PROTEIN-SYNTHESIS
J. Komenda et J. Barber, COMPARISON OF PSBO AND PSBH DELETION MUTANTS OF SYNECHOCYSTIS PCC-6803 INDICATES THAT DEGRADATION OF D1 PROTEIN IS REGULATED BY THE Q(B) SITE AND DEPENDENT ON PROTEIN-SYNTHESIS, Biochemistry, 34(29), 1995, pp. 9625-9631
Mutants of the cyanobacterium Synechocystis PCC 6803 lacking the psbO
or psbH gene are more vulnerable to photoinhibition than the wild type
(WT), In the case of the psbO-less mutant, the increased sensitivity
to photodamage is also accompanied by accelerated turnover of the D1 p
rotein and a rapid rate of recovery on transfer to non-photoinhibitory
conditions. In contrast, in low light the psbH-less mutant has a poor
ability to recover after photoinhibition and has a reduced rate of D1
turnover as compared with WT, Since the psbO gene encodes the 33 kDa
manganese-stabilizing protein associated with the water-splitting reac
tion, the increased sensitivity to photoinduced damage is attributed t
o perturbation of electron transfer processes on the donor side of pho
tosystem II (PSII). In contrast, the absence of H protein, encoded by
the psbH gene, affects the acceptor side of PSII with preferential pho
toinhibitory damage occurring at the Q(B) site. The apparent consequen
ce of this is that the psbH-less mutant, unlike the psbO-less mutant,
is not able to regulate the rate of turnover of the D1 protein. In all
cases it was shown that chloramphenicol, which blocks protein synthes
is, enhances the rate of photoinhibition as judged by a decrease in ox
ygen evolution but slows down the rate of degradation of D1 protein co
mpared to that observed during normal turnover, We conclude either tha
t a factor or enzyme that is rapidly turned over is required to allow
the D1 degradation to occur at in vivo rates or that the degradation a
nd removal of the D1 protein from damaged reaction centers is synchron
ized with the availability of newly synthesized D1 protein. We favor t
he latter on the basis of the relationship between turnover rates and
message level. Our findings also support the concept that D1 turnover
is in some way regulated by the state of the Q(B)-binding pocket.