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Description:
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The ubiquity of circadian systems has allowed their characterization in a broad range of
model systems , which has greatly improved knowledge of how these systems are
organized and the vast range of cellular and organismal processes under circadian
control . Most of the advances , however , have come in describing the central oscillators
of these systems , and , in some cases , the input pathways used to coordinate these
oscillators to external time . Very little progress has been made in understanding the
output pathways that allow circadian systems to regulate the breadth of processes shown
to be clock -controlled .
A genetic selection was designed to obtain mutations in genes involved in circadianregulated
expression of the Neurospora crassa ccg -1 and ccg -2 genes . Some , but not all ,
of the strains obtained display altered regulation of more than one ccg as well as an ‘Easlike’
appearance on solid media , and altered circadian period on race tubes . The data
suggest a model in which output from the clock to these two genes is through a single ,
bifurcated pathway . The cloning of the gene mutated (rrg -1 ) in one of the strains from the above selection
led to the first molecular description of a circadian output pathway in Neurospora , the
HOG MAP kinase pathway . The HOG pathway has been previously described with
regard to its role in the osmotic -stress response . The discovery of the involvement of
rrg -1 in circadian regulation of ccg -1 and ccg -2 led to the discovery of regulation of the
HOG pathway by the circadian clock . The data indicate that osmotic stress information
and time -of -day information are transduced through the HOG pathway and implicate a
role for the clock in preparing the organism for daily occurrences of hyperosmotic stress
associated with sun exposure .
The genetic selection , and the description of the HOG pathway with regard to circadian
output , provide a basis for further characterization of circadian output in Neurospora .
The ubiquity of MAP kinase pathways , such as the HOG pathway , and the observed
similarities in the mechanisms of circadian clock function across multiple phyla , indicate
that these findings may well be applicable to other model systems . |