A 10 residue motif at the C-terminus of the RNA pol II CTD is required for transcription, splicing and 3′ end processing

The RNA polymerase II C-terminal heptad repeat domain (CTD) is essential for normal transcription and co-transcriptional processing of mRNA precursors. The mammalian CTD comprises 52 heptads whose consensus, YSPTSPS, is conserved throughout eukaryotes, followed by a 10 amino acid C-terminal sequence that is conserved only among vertebrates. Here we show that surprisingly, the heptad repeats are not sufficient to support efficient transcription, pre-mRNA processing or full cell viability. In addition to the heptads, the 10 amino acid C-terminal motif is essential for high level transcription, splicing and poly(A) site cleavage. Efficient mRNA synthesis from a transiently transfected reporter gene required the C-terminal motif plus between 16 and 25 heptad repeats from either the N- or C-terminal half of the CTD. Twenty-seven consensus heptads plus the C-terminal motif also supported efficient mRNA synthesis but not cell viability.