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Mol. Cell. Biol., May 1996, 2350-2360, Vol 16, No. 5
EF Michelotti, GA Michelotti, AI Aronsohn and D Levens
The CT element is a positively acting homopyrimidine tract upstream of the
c-myc gene to which the well-characterized transcription factor Spl and
heterogeneous nuclear ribonucleoprotein (hnRNP) K, a less well-
characterized protein associated with hnRNP complexes, have previously been
shown to bind. The present work demonstrates that both of these molecules
contribute to CT element-activated transcription in vitro. The
pyrimidine-rich strand of the CT element both bound to hnRNP K and
competitively inhibited transcription in vitro, suggesting a role for hnRNP
K in activating transcription through this single-stranded sequence. Direct
addition of recombinant hnRNP K to reaction mixtures programmed with
templates bearing single-stranded CT elements increased specific RNA
synthesis. If hnRNP K is a transcription factor, then interactions with the
RNA polymerase II transcription apparatus are predicted. Affinity columns
charged with recombinant hnRNP K specifically bind a component(s) necessary
for transcription activation. The depleted factors were biochemically
complemented by a crude TFIID phosphocellulose fraction, indicating that
hnRNP K might interact with the TATA-binding protein (TBP)-TBP-associated
factor complex. Coimmunoprecipitation of a complex formed in vivo between
hnRNP K and epitope-tagged TBP as well as binding in vitro between
recombinant proteins demonstrated a protein-protein interaction between TBP
and hnRNP K. Furthermore, when the two proteins were overexpressed in vivo,
transcription from a CT element-dependent reporter was synergistically
activated. These data indicate that hnRNP K binds to a specific cis
element, interacts with the RNA polymerase II transcription machinery, and
stimulates transcription and thus has all of the properties of a
transcription factor.
Copyright © 1996, American Society for Microbiology
Heterogeneous nuclear ribonucleoprotein K is a transcription factor
Laboratory of Pathology, National Cancer Institute, National Institute of Health, Bethesda, Maryland 20892, USA.
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