Plant Molecular Biology 37: 1023–1033, 1998.
© 1998 Kluwer Academic Publishers. Printed in Belgium.
Expression of psbA genes produces prominent 5
psbA mRNA fragments in
Synechococcus sp. PCC 7942
A.J. Soitamo, K. Sippola and E.-M. Aro
Department of Biology, University of Turku, BioCity A, Tykistökatu 6, 20520 Turku, Finland (
Received 6 June 1997; accepted in revised form 4 March 1998
Key words: gene expression, psbA gene, truncated psbA messages, DNA-binding protein, D1 protein
Expression of the psbA genes, which in the cyanobacterium Synechococcus sp. PCC 7942 encode two different
forms of the reaction centre D1 protein of photosystem II (D1:1 and D1:2), was studied under different light and
temperature conditions. In addition to the mature 1200 nt psbA messages, three shorter mRNA fragments of 220,
320 and 900 nt were also found. All three mRNA fragments could be recognized by using different gene probes
from the coding region of the psbAI gene, whereas the corresponding psbAII/III gene probes recognized only the
220 nt mRNA fragment. The 5
320 nt mRNA fragment from the psbAI gene probably represents a degradation
product, since the corresponding 3
900 nt psbAI mRNA fragment was also detected. By contrast, the 5
mRNA fragment of all psbA messages is suggested to be a truncated psbA transcript, since no corresponding 3
fragment was ever found. Inhibition of translation either by a protein synthesis inhibitor or by a shift of cells to
lower temperature, increased the number of 1200 nt psbAII/III messages but the number of 5
220 nt psbAII/III
mRNA fragment increased even more dramatically. The ﬁrst 66 bp after ATG, where the psbAI and psbAII/III
genes mostly differ from each other, also appeared important in determining the amount of produced truncated
psbA transcripts, as evidenced by the expression of different tac-psbA constructs in the presence of protein syn-
thesis inhibitor. We suggest that both the psbAI and the psbAII/III genes have a latent intragenic termination site
and truncated psbA transcripts are produced at high levels under stress conditions when transcription becomes
uncoupled from translation.This is to prevent wasting metabolic energy in the production of unused transcripts.
Photosystem II (PSII) is a pigment protein complex in
the thylakoid membrane of higher plants, algae and
cyanobacteria. Despite the complex protein compo-
sition of PSII, only two homologous reaction centre
proteins, the D1 and D2 proteins, bind all the re-
dox active components involved in electron transport
across the thylakoid membrane, from water to the
plastoquinone molecule [18, 31].
In the cyanobacterium Synechococcus sp. PCC
7942 three psbA genes encode two different forms of
D1 protein: form I (D1:1) and form II (D1:2) .
At relatively low growth light intensity, the PSII re-
action centres mainly contain D1:1 protein, encoded
by the psbAI gene, while a shift to high light induces
a rapid replacement of D1:1 with D1:2 which is en-
coded by psbAII and psbAIII genes. This D1 protein
form exchange is preceded by enhanced transcription
of psbAII/III genes, whereas transcription of the ps-
bAI gene is suppressed [23, 24]. Longer exposure of
cells to high irradiance, however, induces the psbAI
messages to become the prevailing form again [8, 14].
Similarly, a shift of Synechococcus cells to low tem-
perature induces a transient replacement of D1:1 with
D1:2 protein in PSII reaction centres .
Three different cis-acting elements have been
found to contribute to the expression of the psbA genes
: a basal promoter comparable to Escherichia coli
promoter, a negative element upstream from the
promoter and a positive element downstream from the
promoter. Further, the high-light induction of the ps-