Plant Molecular Biology 41: 425–433, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Thiol redox state regulates expression of psbA genes in Synechococcus sp.
Katja Sippola and Eva-Mari Aro
Department of Biology, University of Turku, 20014 Turku, Finland (
author for correspondence)
Received 31 May 1999; accepted in revised form 27 August 1999
Key words: D1 protein, gene expression, psbA genes, redox regulation, Synechococcus
Three psbA genes encode two different forms of the photosystem II reaction centre protein D1 in Synechococcus sp.
PCC 7942. The psbAI gene encoding D1 protein form I (D1:1) is mainly expressed under low growth light condi-
tions while the psbAII and psbAIII genes, encoding D1 protein form II (D1:2), are induced under stress conditions
(e.g. high light or low temperature). In this paper we show that psbAII/III genes can be rapidly induced even under
low growth lightconditionsby adding the thiol reductant(DTTred) to Synechococcuscell culture, at a concentration
that does not affect cell growth or photosynthetic activity. Similar induction of psbAII/III genes was obtained by
illuminating the cells with photosystem I light. In both instances psbAI gene down-regulation coincided with the
up-regulation of psbAII/III genes. DTTred-induced exchange in transcript pools was subsequently followed by an
exchange of D1:1 for D1:2 at the protein level. Thiol oxidants, iodosobenzoic acid or diamide, reverted the effects
of DTTred on psbA gene expression. Thiol oxidants and the thiol-modifying agent N-ethylmaleimide also totally
prevented high-light induction of psbAII/III genes. These data strongly suggest that the up-regulation of psbAII/III
genes that occurs under stress conditions is mediated by production of thiol reductants, whereas the expression of
the psbAI gene is sustained by the more oxidizing conditions that prevail during the steady-state growth of cells.
In photosynthetic oxygen-evolving organisms, the
membrane-embedded complexes of photosystem II
(PS II), cytb
/f and photosystem I (PS I) are involved
in the light-driven transfer of electrons from water to
NADP. The reaction centre of PS II contains two struc-
turally similar proteins, D1 and D2, which harbour the
redox components that participate in the primary pho-
tochemical reactions (Barber and Anderson, 1992).
D1 protein is a key element in oxygenic photosynthe-
sis; it is rapidly degraded in the light, with the rate
of degradation being directly proportional to the light
intensity, and it is then replaced by a newly synthe-
sized polypeptide during a PS II repair cycle. Under
environmental stress, the damage and loss of D1 pro-
tein can occur faster than its replacement, leading to
inhibition of PS II (Aro et al., 1993).
Cyanobacterial D1 proteins are encoded by small
psbA gene families, and in Anabaena and Synechococ-
cus, two different forms of D1 protein are produced
(Curtis and Haselkorn, 1984; Jansson et al., 1987).
In the cyanobacterium Synechococcus sp. PCC 7942
(hereafter referred to as Synechococcus) the reaction
centre of PS II may contain either one of the two
forms of D1 protein, D1:1 or D1:2, which differ from
each other in 25 of the total 360 amino acid residues
(Golden et al., 1986). The psbAI gene encoding D1:1
protein is constitutively expressed under normal low-
light growth conditions. Transferringcells to high light
results in accelerated decay of psbAI message and
transcriptional induction of psbAII/III genes, encod-
ing the alternative form of D1 protein, D1:2 (Schaefer
and Golden, 1989; Kulkarni et al., 1992; Clarke
et al., 1993a). It is well documented that acclimation
of Synechococcus cells to low temperatures involves
a similar exchange in psbA transcript pools and D1
protein forms. However, in contrast to the response
of Synechococcus cells to high light, under low-light
conditions, exchange at the protein level lags behind