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Mol. Cell. Biol., 08 1995, 4021-4030, Vol 15, No. 8
CR Kirkpatrick and P Schimmel
The product of the Saccharomyces cerevisiae LEU3 gene, Leu3p, is a
transcriptional activator which regulates leucine biosynthesis in response
to intracellular levels of leucine through the biosynthetic intermediate
alpha-isopropylmalate. We devised a novel assay to examine the DNA site
occupancy of Leu3p under different growth conditions, using a reporter gene
with internal Leu3p-binding sites. Expression of the reporter is inhibited
by binding of nuclear Leu3p to these sites; inhibition is dependent on the
presence of the sites in the reporter, on the integrity of the Leu3p
DNA-binding domain, and, surprisingly, on the presence of a transcriptional
activation domain in the inhibiting protein. By this assay, Leu3p was found
to occupy its binding site under all conditions tested, including high and
low levels of leucine and in the presence and absence of
alpha-isopropylmalate. The localization of Leu3p to the nucleus was
confirmed by immunofluorescence staining of cells expressing epitope-tagged
Leu3p derivatives. We conclude that Leu3p regulates transcription in vivo
without changing its intracellular localization and DNA site occupancy.
Copyright © 1995, American Society for Microbiology
Detection of leucine-independent DNA site occupancy of the yeast Leu3p transcriptional activator in vivo
Department of Biology, Massachusetts Institute of Technology, Cambridge 02139, USA.
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