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Description:
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Cell cycle progression of Saccharomyces cerevisiae cells was monitored in
continuous cultures limited for glucose or nitrogen . The G1 cell cycle phase , before
initiation of DNA replication , did not exclusively expand when growth rate decreased .
Especially during nitrogen limitation , non -G1 phases expanded almost as much as G1 . In
addition , cell size remained constant as a function of growth rate . These results contrast
with current views that growth requirements are met before initiation of DNA replication ,
and suggest that distinct nutrient limitations differentially impinge on cell cycle
progression . Therefore , multiple mechanisms are hypothesized to regulate the
coordination of cell growth and cell division .
Genetic interactions were identified between the dose -dependent cell -cycle
regulator 2 (DCR2 ) phosphatase and genes involving in secretion /unfolded protein
response pathway , including IRE1 , through a genome -wide dominant negative genetic
approach . Accumulation of unfolded proteins in the endoplasmic reticulum triggers the
unfolded protein response (UPR ) . How the UPR is downregulated is not well
understood . Inositol requirement 1 (IRE1 ) is an endoplasmic reticulum transmembrane UPR sensor in Saccharomyces cerevisiae . When the UPR is triggered , Ire1p is
autophosphorylated , on Ser 840 and Ser 841 , inducing the cytosolic endonuclease
activity of Ire1p , thereby initiating the splicing and translational de -repression of HAC1
mRNA . Homologous to Atf /Creb1 (Hac1p ) activates UPR transcription . We found that
that Dcr2p phosphatase functionally and physically interacts with Ire1p . Overexpression
of DCR2 , but not of a catalytically inactive DCR2 allele , significantly delays HAC1
splicing and sensitizes cells to the UPR . Furthermore , Dcr2p physically interacts in vivo
with Ire1p -S840E , S841E , which mimics phosphorylated Ire1p , and Dcr2p dephosphorylates
Ire1p in vitro . Our results are consistent with de -phosphorylation of
Ire1p being a mechanism for antagonizing UPR signaling . |