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Mol. Cell. Biol., Jan 1997, 100-114, Vol 17, No. 1
LM Facchini, S Chen, WW Marhin, JN Lear and LZ Penn
Increasing evidence supports an important biological role for Myc in the
downregulation of specific gene transcription. Recent studies suggest that
c-Myc may suppress promoter activity through proteins of the basal
transcription machinery. We have previously reported that Myc protein, in
combination with additional cellular factors, suppresses transcription
initiation from the c-myc promoter. To characterize the cis components of
this Myc negative autoregulation pathway, fragments of the human c-myc
promoter were inserted upstream of luciferase reporter genes and assayed
for responsiveness to inducible MycER activation in Rat-1 fibroblasts. We
found four- to fivefold suppression of a c-myc P2 minimal promoter fragment
upon induction of wild-type MycER protein activity, while induction of a
mutant MycER protein lacking amino acids 106 to 143 required for Myc
autosuppression failed to elicit this response. This assay is
physiologically significant, as it reflects Myc autosuppression of the
endogenous c-myc gene with regard to kinetics, dose dependency, cell type
specificity, and c-Myc functional domains. Analysis of mutations within the
P2 minimal promoter indicated that the cis components of Myc
autosuppression could not be ascribed to any known protein-binding motifs.
In addition, to address the trans factors required for Myc negative
autoregulation, we expressed MycEG and MaxEG leucine zipper dimerization
mutants in Rat-1 cells and found that Myc-Max heterodimerization is
obligatory for Myc autosuppression. Two models for the Myc autosuppression
mechanism are discussed.
Copyright © 1997, American Society for Microbiology
The Myc negative autoregulation mechanism requires Myc-Max association and involves the c-myc P2 minimal promoter
Department of Microbiology, University of Toronto, Ontario, Canada.
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