Plant Molecular Biology 40: 737–744, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Characterisation of the PsbX protein from Photosystem II and light
regulation of its gene expression in higher plants
, Soo J. Kim
, Alan Marchant
, Colin Robinson
and Wolfgang P. Schröder
Department of Biochemistry, Arrhenius Laboratories for Natural Sciences, Stockholm University, 106 91 Stock-
Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK;
address: Södertörn University College, Department of Medical Nutrition, Novum, 141 86 Huddinge, Sweden
author for correspondence)
Received 17 December 1998; accepted in revised form 30 April 1999
Key words: 4.1 kDa protein, Arabidopsis thaliana, low-molecular-mass proteins, photosystem II, PsbX, spinach
The location and expression of the previously uncharacterised photosystem II subunit PsbX have been analysed in
higher plants. We show that this protein is a component of photosystem II (PSII) core particles but absent from
light-harvesting complexes or PSII reaction centres. PsbX is, however, localised to the near vicinity of the reaction
centre because it can be cross-linked to cytochrome b559, which is known to be associated with the D1/D2 dimer.
We also show that the expression of this protein is tightly regulated by light, since neither protein nor mRNA is
found in dark-grown plants.
Abbreviations: Chl, chlorophyll; cyt b559, cytochrome b559; PSII, photosystem II; PSI, photosystem I; LHC,
light-harvesting complex;EDC, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
Photosystem II (PSII) of higher plants catalyses the
light-driven oxidation of water to molecular oxygen
and the reduction of plastoquinone to plastoquinol.
The PSII membrane complex consists of more than
25 different subunits (for reviews, see Andersson and
Franzén, 1992; Pakrasi, 1995). The D1 and D2 pro-
teins have been found to bind most, if not all, of
the cofactors needed for the primary electron transfer
reactions, and these two proteins constitute the PSII
reaction centre. However, no oxygen-evolving active
D1-D2 heterodimer has so far been isolated, which
suggests that additional proteins are needed for the
PSII complex to retain this activity. Some of these an-
cillary proteins have been identiﬁed and characterised
to varying extents, but the true complexity of PSII re-
mains to be established. This applies particularly to a
series of low-molecular-mass proteins that were ﬁrst
identiﬁed using high-resolution SDS-PAGE of vari-
ous PSII samples by Ljungberg et al. (1986). Partial
N-terminal sequences were obtained from several of
these bands (Ikeuchi et al., 1988, 1989; Schröder
et al., 1988)andthree ofthese protein sequences could
not be found in the chloroplast genome, implying that
they were nuclear-encoded.
The largest of these three proteins, the 6.1 kDa
protein (PsbW), has recently been isolated, cloned and
characterised in spinach (Irrgang et al., 1995; Lor-
c et al., 1995; Shi and Schröder et al., 1997).
Interestingly, the PsbW protein was also found to be
degraded upon high light treatments, i.e. photoinhi-
bition conditions (Hagman et al., 1997). The second
protein, the 5.0 kDa protein (PsbT), has been isolated,
cloned and found to be located on the lumenal side
of the thylakoid membrane (Kapazoglou et al., 1995).
Genes encoding both of these proteins were found to
be located in the nucleus as predicted.
Of the three nuclear-encoded small proteins, the
4.1 kDa protein (PsbX) is the least well characterised.
However, a cDNA encoding the Arabidopsis thaliana
protein was recently characterised (Kim et al., 1996)