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Myc correlates globally with transcription complexes rather than specific DNA elements

ABSTRACT: The Myc-Max heterodimer has been thought of as a sequence specific DNA binding protein that regulates transcription of a large number of genes. We demonstrate here that the positions of the human genome occupied by Myc-Max correlate with the RNA polymerase II (Pol II) transcription complex rather than to the canonical DNA binding sequence element CACGTG. The heterodimer is positioned slightly upstream of essentially all promoter proximal paused polymerases and is found throughout the transcribed regions of genes. Using a multi-genome analysis of promoter regions we show that the heterodimers are oriented with respect to the direction of transcription with Myc downstream of Max. In strong support of a model in which the Myc is recruited by transcription complexes rather than specific DNA sequences, we found that the difference in affinities of Myc-Max heterodimers for CACGTG versus non-specific DNA is not great enough to drive the pattern of genome occupancy exhibited.

ORGANISM(S): Homo sapiens

PROVIDER: GSE43227 | GEO | 2013/01/02

SECONDARY ACCESSION(S): PRJNA185044

REPOSITORIES: GEO

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Project description:The Myc-Max heterodimer is a DNA binding protein that regulates expression of a large number of genes. Genome occupancy of Myc-Max is thought to be driven by E-boxes (CACGTG or variants) to which the heterodimer binds in vitro. By analyzing ChIP-Seq datasets, we demonstrated that the positions occupied by Myc-Max across the human genome correlate with the RNA polymerase II (Pol II) transcription machinery better than with E-boxes. Metagene analyses showed that in promoter regions, Myc was uniformly positioned about 100 bp upstream of essentially all promoter proximal paused polymerases with Max about 10 bp upstream of Myc. We re-evaluated the DNA binding properties of full length Myc-Max proteins using electrophoretic mobility shift assays (EMSA) and protein-binding microarrays (PBM). EMSA results demonstrated Myc-Max heterodimers have high affinity for both E-box containing and non-specific DNA. Quantification of the relative affinities of Myc-Max for all possible 8- mers using PBM assays showed that sequences surrounding core 6-mers significantly affect binding. Comparing to the in vitro sequence preferences, Myc-Max genomic occupancy measured by ChIP-Seq was largely, although not completely, independent of sequence specificity. Our results suggest that the transcription machinery and associated promoter accessibility play an important role in genomic occupancy of Myc. Two protein binding microarray (PBM) experiments were performed: one for the heterodimer of the human transcription factors c-Myc and Max, and one for the Max-Max homodimer. Briefly, 4x44K arrays (Agilent Technologies; AmadID 015681) containing the M-bM-^@M-^Xall 10-merM-bM-^@M-^Y universal PBM design were used. Arrays were incubated with a PBS buffer based protein mixture of wither 10nM His-tagged Myc-Max heterodimer or 10nM His-tagged Max-Max homodimer, 2% milk, 200ng/M-BM-5L BSA, 50ng/M-BM-5L Salmon Testes DNA, and 0.02% TX-100. Bound protein was tagged with 10ng/M-BM-5L anti-His antibody conjugated to Alexa 488 (Qiagen; 35310) in PBS with 2% milk. Data were analyzed to obtain fluorescence intensities for all 8mers. The PBM protocol is described in Berger et al., Nature Biotechnology 2006 (PMID 16998473).

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Myc correlates globally with transcription complexes rather than specific DNA elements

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