Plant Molecular Biology 50: 573–585, 2002.
© 2002 Kluwer Academic Publishers. Printed in the Netherlands.
Light activates binding of membrane proteins to chloroplast RNAs in
, Shengwu Wang
and Jean-David Rochaix
Biology Department, Concordia University, 1445 Maisonneuve W., Montreal, Canada, H3G 1M8 (
for correspondence; email firstname.lastname@example.org);
Departments of Molecular Biology and Plant Biology,
University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
Received 4 December 2001; accepted in revised form 14 May 2002
Key words: chloroplast, light, organelle, photosynthesis, RNA, RNA-binding protein
Several membrane proteins were previously shown to bind to the 5
leader of the chloroplast psbC mRNA in the
unicellular eukaryotic alga Chlamydomonas reinhardtii. This study showed that these proteins have afﬁnity for
AU-rich RNAs, as determined by competition experiments. In addition, their binding activities are enhanced 13–
15-fold by light, and a 46 kDa protein is activated within 1–10 min. This activation could be mediated by the
modulation of ADP pools by the light-dependent reactions of photosynthesis and ATP synthase because (1) two
inhibitors that block ATP synthesis also prevent this activation and (2) ADP inhibits the RNA-binding activity of
this protein in vitro. An inhibitor of Photosystem II diminishes this induction, suggesting that reducing potential
generated by the photosynthetic electron transport chain modulates this RNA-binding activity. The RNA-binding
activities of two proteins (of 46 and 47 kDa) are inhibited by Mg-protoporphyrin IX methyl ester in vitro suggesting
they could be regulated by these intermediates in the chlorophyll biosynthetic pathway.
Abbreviations: DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone; DCMU, 3-(3,4-dichlorophenyl)-1,1-
dimethyl urea; FCCP, carbonyl cyanide p-(triﬂuoromethoxy)phenylhydrazone; MgPROTO, Mg-protoporphyrin
IX; MgPROTOMe, Mg-protoporphyrin IX dimethyl ester; PAGE, polyacrylamide gel electrophoresis; RBP, RNA-
binding protein; TAP, Tris-acetate-phosphate medium
Plastids have remnants of the genetic system of their
ancestral eubacterial photosynthetic endosymbiont.
Circular plastid genomes encode many of the polypep-
tide subunits of the photosynthetic apparatus and
components of the organellar gene expression system
(Gray, 1999). Most plastid proteins are encoded by
nuclear genes, translated on 80S cytosolic ribosomes,
and then imported into the organelle (Caliebe and
Soill, 1999). Chloroplast gene expression is regulated
by light intensity (Christopher et al., 1997), develop-
mental programs (Inada et al., 1996) and circadian
rhythms (Hwang et al., 1996), and occurs at multiple
levels: transcription (Mullet, 1993; Christopher et al.,
1997), mRNA stability (Nickelsen, 1998), mRNA
translation (Rochaix, 1996; Hauser et al., 1998; Sug-
iura et al., 1998; Zerges, 2000, 2002), and protein
degradation (Adam, 2000; Choquet and Vallon, 2000).
Translation is a particularly important level of con-
trol, and requires interactions between chloroplast
mRNAs and nucleus-encoded factors (Barkan and
Goldschmidt-Clermont, 2000; Zerges, 2000, 2002).
Processing events to form the 3
terminus and, possi-
bly, the 5
terminus might be required for translation
(Rott et al., 1998; Monde et al., 2000). In the eu-
karyotic green alga Chlamydomonas reinhardtii, light
stimulates synthesis of the polypeptides encoded by
the chloroplast psbA and psbD genes (Malnoe et al.,
1988) and transiently decreases synthesis of the large
subunit of bisphosphate carboxylase (LSU), encoded
by the chloroplast rbcL gene (Shapira et al., 1997).