(a) In resting cells, which do not express MYC, the transcription elongation
factor SPT5 is insufficiently recruited to RNA polymerase II (Pol II), which
loses directionality and processivity, resulting in increase in the levels of
antisense and abortive transcripts. (b) In growing cells, MYC is expressed,
binds SPT5 and recruits it to promoters. The transfer of SPT5 from MYC to Pol II
depends on cyclin-dependent kinase 7 (CDK7).Pol II associated with transcription
elongation factors engages in productive (fast, processive and directional)
transcription elongation and produces full-length mRNAs. (c) In cancer cells
expressing high levels of MYC, a considerable fraction of SPT5 is sequestered by
soluble MYC, and transcription is decreased at known MYC-repressed genes. TSS,
transcription start site.