Plant Molecular Biology 39: 1013–1023, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
PTK, the chloroplast RNA polymerase-associated protein kinase from
mustard (Sinapis alba), mediates redox control of plastid in vitro
Sacha Baginsky, Kai Tiller, Thomas Pfannschmidt and Gerhard Link
University of Bochum, Plant Cell Physiology and Molecular Biology, Universitätsstrasse 150, 44780 Bochum,
author for correspondence)
Received 30 July 1988; accepted in revised form 29 November 1998
Key words: chloroplast gene expression, DNA-dependent RNA polymerase, SH-group control, transcription
kinase, organelle signal transduction
The major RNA polymerase from mustard chloroplasts is a multi-subunit enzyme consisting of core components
and associated factors. Among the latter is a heterotrimeric factor named PTK (plastid transcription kinase) because
of its serine/threonine-type protein kinase activity. PTK activity itself depends on its phosphorylation state. In
addition, we show that it responds to glutathione but not to other redox-reactive reagents that were tested, and
both glutathione and phosphorylation act antagonistically. Using a homologous in vitro system, we ﬁnd that PTK
selectively phosphorylatessubunit(s) of plastid RNA polymerase and is involved in determining the level of faithful
transcription from the chloroplast psbA promoter. Together, these results establish a role for phosphorylation and
redox state in the regulation of plastid gene expression.
Chloroplasts, the sites of photosynthesis and major
biosynthetic pathways, have their own gene expres-
sion system (for recent reviews, see e.g. [22, 46,
50]), and both post-transcriptional and transcriptional
mechanisms participate in the regulation of organellar
gene expression during development and in response
to environmental cues [38, 41, 49]. Photosynthesis
and plastid gene expression not only operate in phys-
ical proximity but are also functionally connected.
Photosynthetic proteins are degraded during normal
photosynthesis and – dramatically accelerated – un-
der high-light and oxidative stress conditions [3, 5,
37, 42]. As there is a continuous need for replenish-
ment , this will require balanced re-synthesis and
speciﬁc signalling mechanisms.
Although the current picture of the cross-talk be-
tween photosynthesis and gene expression is far from
This paper is dedicated to Professor Achine Trebst on the
occasion of his 70th birthday.
complete, recent data indicate that chloroplast gene
expression is controlled by mechanisms that play key
regulatory roles in photosynthesis, i.e. protein phos-
phorylation and (SH-group) redox poise [32, 39].
Their function in post-transcriptional steps of plastid
gene expression is well established in the case of an
RNA-binding compleximplicated in translational con-
trol via binding to the 5
regionof psbA mRNA [11, 12,
29, 56]. In addition, evidence has been obtained that
an endoribonuclease involved in the 3
ing of chloroplast RNA precursors is also controlled
both by phosphorylationand and SH-group redox state
With regard to chloroplast transcription, less direct
evidence for concerted regulation by phosphorylation
and redox poise has been available so far. This can
be attributed in part to the fact that important details
of the transcription apparatus have only recently been
resolved. At least two distinct RNA polymerases exist
in chloroplasts. One is of the eubacterial multi-subunit
type, with core components encoded by chloroplast