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Description:
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The circadian input kinase gene (cikA ) was first identified from a Tn5 mutant
of Synechococcus elongatus PCC 7942 . A cikA null strain shows a striking
phenotype related to circadian gene regulation : all sampled loci show a
shortened circadian period and reduced amplitude of oscillation and a failure to
exhibit a wild -type resetting of the phase of the rhythm after an environmental
signal . This global defect in response to the environment suggests a key role for
CikA in the circadian input pathways . Bioinformatics results classify CikA as a
divergent member of the bacteriophytochrome family , suggesting a role in light
signal transduction . In vitro analysis previously showed that CikA is a bona fide
histidine protein kinase (HPK ) , and its kinase activity is regulated by the
presence of other domains . Its own pseudo -receiver (PsR ) domain is not the
cognate receiver domain of its kinase HPK domain , and its GAF domain does
not likely bind a bilin chromophore as do photoreceptive phytochromes . Recent
results suggested that CikA may function as a redox -sensor . In this study , we examined the function of each domain of CikA using
different mutant cikA alleles , and determined their phenotypes with respect to
complementation of a null mutant and overexpression in both wild type and cikA
null strains . All domains except the featureless N -terminus were required for
CikA function . Overexpression of all mutant alleles that encoded the PsR
domain , whether or not the HPK was functional , caused a dominant arrhythmia
phenotype . In the absence of PsR , overexpressed variants did not cause
arrhythmia , but affected the amplitude and period of oscillation . The results
suggest a model in which the PsR domain regulates kinase activity and
mediates interaction with other input pathway components to allow CikA to reach
the correct cellular position to fulfill its function . Cellular localization assays
showed CikA can interact with a complex and showed a polar localization
pattern , whereas its variant without PsR showed uniform distribution in the cell .
In summary , CikA is an autoregulated kinase in which the PsR domain
regulates activity of the HPK domain and also serves as an interaction module to
lead the CikA to a specific cellular position . |