Anti-terminator protein required to prevent early mRNA termination during transcription (PubMed:31104839). Together with SCAF4, acts by suppressing the use of early, alternative poly(A) sites, thereby preventing the accumulation of non-functional truncated proteins (PubMed:31104839). Mechanistically, associates with the phosphorylated C-terminal heptapeptide repeat domain (CTD) of the largest RNA polymerase II subunit (POLR2A), and subsequently binds nascent RNA upstream of early polyadenylation sites to prevent premature mRNA transcript cleavage and polyadenylation (PubMed:31104839). Independently of SCAF4, also acts as a positive regulator of transcript elongation (PubMed:31104839). Interacts with POLR2A; via C-terminal heptapeptide repeat domain (CTD) phosphorylated at 'Ser-2' and 'Ser-5' (PubMed:18550522, PubMed:9528809, PubMed:31104839). Identified in a complex with CDC5L and other spliceosomal proteins (PubMed:11101529). Detected in granular nuclear foci which correspond to sites of active transcription.