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Mol. Cell. Biol., 02 1996, 634-647, Vol 16, No. 2
J Wells, P Held, S Illenye and NH Heintz
In mammals, two TATA-less bidirectional promoters regulate expression of
the divergently transcribed dihydrofolate reductase (dhfr) and rep3 genes.
In CHOC 400 cells, dhfr mRNA levels increase about fourfold during the
G1-to-S phase transition of the cell cycle, whereas the levels of rep3
transcripts vary less than twofold during this time. To assess the role of
DNA-binding proteins in transcriptional regulation of the dhfr and rep3
genes, the major and minor dhfr-rep3 promoter regions were analyzed by
high-resolution genomic footprinting during the cell cycle. At the major
dhfr promoter, prominent DNase I footprints over four upstream Sp1 binding
sites did not vary throughout G1 and entry into the S phase. Genomic
footprinting revealed that a protein is constitutively bound to the
overlapping E2F sites throughout the G1-to-S phase transition, an
interaction that is most evident on the transcribed template strand. On the
nontranscribed strand, multiple changes in the DNase I cleavage pattern are
observed during transit through G1 and entry into the S phase. By using gel
mobility shift assays and a series of sequence-specific probes, two
different species of E2F were shown to interact with the dhfr promoter
during the cell cycle. The DNA binding activity of one E2F species, which
preferentially recognizes the sequence TTTGGCGC, did not vary significantly
during the cell cycle. The DNA binding activity of the second E2F species,
which preferentially recognizes the sequence TTTCGCGC, increased during the
G1-to-S phase transition. Together, these results indicate that Sp1 and the
species of E2F that binds TTTGGCGC participate in the formation of a basal
transcription complex, while the species of E2F that binds TTTCGCGC
regulates dhfr gene expression during the G1-to-S phase transition. At the
minor promoter, DNase I footprints at a consensus c-Myc binding site and
three Sp1 binding sites showed little variation during the G1-to-S phase
transition. In addition to protein binding at sequences known to be
involved in the regulation of transcription, genomic footprinting of the
entire promoter region also showed that a protein factor is constitutively
bound to the first intron of the rep3 gene.
Copyright © 1996, American Society for Microbiology
Protein-DNA interactions at the major and minor promoters of the divergently transcribed dhfr and rep3 genes during the Chinese hamster ovary cell cycle
Program in Cell and Molecular Biology, University of Vermont College of Medicine, Burlington 05405, USA.
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