Most transcription factors specify the subset of genes that will be actively transcribed in the cell by stimulating transcription initiation at these genes, but MYC has a fundamentally different role. MYC binds E-box sites in the promoters of active genes and stimulates recruitment of the elongation factor P-TEFb and thus transcription elongation. Consequently, rather than specifying the set of genes that will be transcribed in any particular cell, MYC's predominant role is to increase the production of transcripts from active genes. This increase in the transcriptional output of the cell's existing gene expression program, called transcriptional amplification, has a profound effect on proliferation and other behaviors of a broad range of cells. Transcriptional amplification may reduce rate-limiting constraints for tumor cell proliferation and explain MYC's broad oncogenic activity among diverse tissues.
TRANSCRIPTIONAL REGULATIONT ranscription factors bind specific DNA sequences and regulate the recruitment and activity of the transcription apparatus at genes (Ptashne and Gann 1997; Lee and Young 2013). The process of transcription consists of at least three steps: initiation, elongation, and termination (Fuda et al. 2009;Malik and Roeder 2010;Zhou et al. 2012). During initiation, the transcription apparatus, which consists of RNA polymerase II (Pol II) and various cofactors, is recruited to genes by transcription factors. A short transcript is produced by Pol II and pause factors typically induce pausing 20 -50 bp downstream of the transcriptional start site. Elongation proceeds after the elongation factor P-TEFb, which consists of Cdk9 and cyclin T, is recruited, and phosphorylates the pause factors and Pol II. Transcription termination is stimulated by recognition of polyadenylation site sequences by factors associated with Pol II during elongation.It has long been clear that specific transcription factors are responsible for recruiting Pol II to selected genes during transcription initiation, but evidence emerged in the last decade that argues for an additional level of control at the pause-release and/or elongation stage of transcription for a large number of genes (Fuda et al. 2009;Nechaev and Adelman 2011;Zhou et al. 2012;Conaway and Conaway 2013). For example, in various human cells, Pol II was found to occupy the promoters of the majority ( 70%) of protein-coding genes, but full-length transcripts were detected at only a subset of these genes (Guenther et al. 2007). Similarly, a large fraction of Drosophila genes with roles in devel-