The carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II (pol 
II) is essential for several co-transcriptional pre-messenger RNA processing 
events, including capping, 3'-end processing and splicing. We investigated the 
role of the CTD of RNA pol II in the coordination of A to I editing and splicing 
of the ADAR2 (ADAR: adenosine deaminases that act on RNA) pre-mRNA. The 
auto-editing of Adar2 intron 4 by the ADAR2 adenosine deaminase is tightly 
coupled to splicing, as the modification of the dinucleotide AA to AI creates a 
new 3' splice site. Unlike other introns, the CTD is not required for efficient 
splicing of intron 4 at either the normal 3' splice site or the alternative site 
created by editing. However, the CTD is required for efficient 
co-transcriptional auto-editing of ADAR2 intron 4. Our results implicate the CTD 
in site-selective RNA editing by ADAR2 and in coordination of editing with 
alternative splicing.