Research
Research.
 | Plant Cell Biology, September 1995. From a Kodak
photo CD. All friends; all colleagues. These personal pages are just a subset of
the Plant Cell Biology web site. From left to right: Paul Davison;
Carin Jarl; Dalibor Stys; Ineke de Jong; Annalisa Svensson; Lüling Cheng;
Anders Kylin; Martha Escobar; Krassimir Alexciev; Allen (in disguise); Anna
Tullberg. André Struglics took the photograph. Carol and Gunilla were not
around. They can all be seen in the main pages.
|
A lot of information here, organised into:
Overview of areas of research
Superoxide and oxygen reduction
Protein phosphorylation
Current work and future directions
Sex
Photosynthesis; oxygen reduction; photosynthetic electron transport and ATP synthesis; protein phosphorylation, light-
harvesting, chlorophyll fluorescence and excitation energy transfer in chloroplasts and photosynthetic prokaryotes; state 1-
state 2 transitions; molecular recognition and structural effects of protein phosphorylation; redox control of gene expression;
evolution of chloroplast and mitochondrial genomes and control of gene expression; evolution of sex; ageing.
Contributions to knowledge have been made in three distinct areas. The first two concern photosynthesis, a process of
fundamental biological importance by which plants and some bacteria are able to harness light energy in a form that is
chemically stable and therefore biologically useful. The third links electron transport in photosynthesis and respiration to
gene expression, and offers explanations of the evolution of cytoplasmic genetic systems and sex.
The first area concerns the chemical reduction of oxygen by photosynthetic systems. Here I provided the first experimental
evidence for the existence of the toxic superoxide radical as an intermediate in photosynthetic oxygen reduction by isolated
chloroplasts, the subcellular organelle in which photosynthesis occurs in green plants. I was able to propose a physiologically
safe mechanism for oxygen reduction via superoxide by the electron carrier ferredoxin, and to examine the possible roles of
oxygen reduction in the energy metabolism in plant cells. The work has additional implications for herbicide action, oxygen
toxicity and iron-sulphur proteins. These studies occupied me during the period 1972-78, as a postgraduate (King's College
London) and in my first postdoctoral position (Oxford). My findings have been largely substantiated and have provided the
basis for subsequent research, as indicated by continued citation of my early publications and their coverage in review
articles.
In 1979 I started a second postdoctoral position (Warwick) and embarked on research in a different area. This area is
concerned with biochemical and physiological regulation of the transfer of absorbed excitation energy and its distribution
between the photochemical reaction centres that are the primary traps for conversion of light energy into electron transport
and hence into stored chemical potential energy. In 1980 I discovered that the modification, by protein phosphorylation, of a
light harvesting chlorophyll-protein complex is itself controlled by the state of oxidation-reduction of a particular electron
carrier, plastoquinone. This allowed my coworkers and me to propose a feedback mechanism to explain a well-known
adaptation of green plants to changing light regimes. This work has attracted considerable interest and support and has
provided a discernable theme in areas of photosynthesis research during the 1980s.
One central objective of my recent work has been to generalize this mechanism for photosynthetic bacteria of several kinds
(notably the purple non-sulphur bacteria and the cyanobacteria) and thereby to view green plant photosynthesis as an
evolutionary special case in respect of regulation of photosynthetic unit function. The results obtained support a general
model for control of photosynthetic unit function. This work is continuing with recent identification of bacterial
phosphoproteins by amino acid sequencing.
I view post-translational control of photosynthetic unit function as an example of altered molecular recognition, and propose
that protein phosphorylation causes regulatory structural changes, in contrast to the widely-accepted model of altered
membrane surface charge. Recent results in my laboratory and in collaboration with Lund Physical Chemistry 2 show clearly
by NMR spectroscopy that phosphorylation causes a major structural change in polypeptides corresponding to the N-terminal
domain of the chloroplast light-harvesting protein. The molecular recognition hypothesis has implications not only
for photosynthetic energy transduction but for the wider field of regulation of membrane protein interactions in biology. I
intend to probe alteration of membrane protein structure and function by a combination of high-resolution structural studies,
fast kinetic fluorescence and absorption spectroscopy, and directed mutagenesis to alter functional and regulatory sites.
Another recent development relates the triggers and mechanism of this post-translational, physiological adaptation to those of
developmental changes in gene expression that operate on longer time-scales. We have recently discovered redox-controlled
modification of a transcriptional activator in cyanobacteria, thereby establishing the possibility of such a link between post-
translational and transcriptional levels of control. I have recently put forward the hypothesis that redox control of gene
expression explains, in principle, the function of the genomes of chloroplasts and mitochondria and their retention, in
evolution, as extra-nuclear genetic systems. If correct, this provides a solution to a long-standing problem in evolutionary cell
biology, namely, why do chloroplasts and mitochondria contain distinct genetic systems to express a small but constant sub-
set of their own proteins? This hypothesis seeks to explain what these proteins have in common that confers a selective
advantage to the location of their genes in situ in the organelle. This is a testable hypothesis, and preliminary results
show specific redox regulation of the pattern of protein synthesis in isolated chloroplasts and mitochondria, consistent with
its predictions. I have also put forward a nomenclature for the components of the two distinct systems that are emerging as
regulatory mechanisms linking electron transport to transcription of specific genes in bacteria.
Redox control of chloroplast
and mitochondrial gene expression locks these genes into the eukaryotic cell's most hostile internal compartments. Fidelity of
their replication may be nevertheless secured by sex, a functional division of labour whereby organelles are adapted either to
energy transduction or, in the female germ line, to replication. Male is that sex in which germ line mitochondria
perform oxidative phosphorylation. Female is that sex in which they do not.
The hypotheses of guided molecular recognition of photosynthetic membrane protein complexes and of redox-linked
translational and transcriptional control are the focus of my current research objectives. Their development and experimental
test comprise my major research priority for the future.
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