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Mol. Cell. Biol., Sep 1997, 4933-4947, Vol 17, No. 9
P Moffett, M Reece and J Pelletier
The Drosophila single-minded (Dsim) gene encodes a master regulatory
protein involved in cell fate determination during midline development.
This protein is a member of a rapidly expanding family of gene products
possessing basic helix-loop-helix (bHLH) and hydrophobic PAS (designated a
conserved region among PER, ARNT [aryl hydrocarbon receptor nuclear
translocator] and SIM) protein association domains. Members of this family
function as central transcriptional regulators in cellular differentiation
and in the response to environmental stimuli such as xenobiotics and
hypoxia. We have previously identified a murine member of this family,
called mSim-2, showing sequence homology to the bHLH and PAS domains of
Dsim. Immunoprecipitation experiments with recombinant proteins indicate
that mSIM-2 associates with the arnt gene product. In the present work, by
using fine- structure mapping we found that the HLH and PAS motifs of both
proteins are required for optimal association. Forced expression of
GAL4/mSIM-2 fusion constructs in mammalian cells demonstrated the presence
of two separable repression domains within the carboxy terminus of mSIM-2.
We found that mSIM-2 is capable of repressing ARNT-mediated transcriptional
activation in a mammalian two-hybrid system. This effect (i) is dependent
on the ability of mSIM-2 and ARNT to heterodimerize, (ii) is dependent on
the presence of the mSIM-2 carboxy- terminal repression domain, and (iii)
is not specific to the ARNT activation domain. These results suggest that
mSIM-2 repression activity can dominantly override the activation potential
of adjacent transcription factors. We also demonstrated that mSIM-2 can
functionally interfere with hypoxia-inducible factor 1alpha (HIF-
1alpha)/ARNT transcription complexes, providing a second mechanism by which
mSIM-2 may inhibit transcription.
Copyright © 1997, American Society for Microbiology
The murine Sim-2 gene product inhibits transcription by active repression and functional interference
Department of Biochemistry, McGill University, Montreal, Quebec, Canada.
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