The heptad repeating sequence of the Cterminal domain (CTD) of the largest subunit of RNA polymerase II is highly conserved in eukaryotes. In yeast, a CTD code consisting of pairs of heptad repeats is essential for viability. However, the strict requirement of diheptad repeats for the CTD function in transcription and splicing is unexplained. Here we show that CoAA (gene symbol RBM14), an oncoprotein and mammalian transcriptional coactivator, possesses diheptad repeats and directly interacts with the CTD. CoAA comprises 27 copies of tyrosine-rich repeats and regulates pre-mRNA synthesis and alternative splicing. Tyrosine substitutions in either the CoAA repeats or the CTD repeats diminish their interactions. Ser2or Ser5-phosphorylated CTD peptides exhibit higher binding affinity to CoAA than the corresponding non-phosphorylated CTD peptide. CoAA dynamically interacts with both the CTD and hnRNP M, which is an alternative splicing regulator also comprising diheptad repeats. Arginine methylation of CoAA switches its interaction from the hnRNP M repeats to the CTD repeats. This study provides a mechanism for CoAA at the interface of transcription and alternative splicing, and explains the functional requirement of diheptad repeats in the CTD. In the human genome, tyrosine-rich repeats similar to the CoAA repeats were only found in six oncoproteins including EWS and SYT. We suggest that the diheptad sequence is one of the signature features for the CTD interaction among oncoproteins involved in transcription and alternative splicing. We anticipate that direct RNA Pol II interaction is a mechanism in oncogenesis.