| dc.description.abstract | Two-component signal transduction, comprising sensor kinases and response
regulators, is the predominant signalling mechanism in prokaryotes. This signalling
system originated in bacteria, and has spread to the eukaryotic domain of life through
symbiotic, lateral gene transfer from the bacterial ancestors of chloroplasts and
mitochondria. During the course of their evolution, chloroplasts, with the exception
of a few instances in non-green algae, appear to have relinquished all genes encoding
two-component systems to their eukaryotic host cell nuclei. In green algae and
plants, chloroplast genes for two-component systems were neither known nor were
chloroplast two-component proteins shown to exist as products of nuclear genes
prior to the work described here. This thesis describes the identification and
characterisation of a novel two-component sensor kinase in chloroplasts. This
Chloroplast Sensor Kinase (CSK) is the product of a nuclear gene in algae and plants.
CSK is synthesised in the cytosol of Arabidopsis thaliana and imported into the
chloroplast as a protein precursor. CSK is autophosphorylated and couples
photosynthetic electron transport to gene transcription in chloroplasts. The identity of
the response regulator partner of CSK reveals an unexpected phylogenetic and
functional relatedness of CSK with chloroplast two-component systems of non-green
algae. Chloroplast two-component systems are likely to be universal in
photosynthetic eukaryotes and they persist in chloroplasts as products of nuclear
genes even where chloroplast genomes no longer encode them. Chloroplast twocomponent
systems have homologues in extant cyanobacterial lineages, indicating
their ancient cyanobacterial origin. The persistence of cyanobacterial two-component
systems in chloroplasts and their function in coupling photosynthesis with
chloroplast gene expression are central to the premise that chloroplasts retain genes
whose expression is regulated by the activity of the photosynthetic electron transport
chain, using a mechanism conserved from their cyanobacterial ancestors. | en_US |
