Functions as a general transcription factor playing a role in the process of transcriptional elongation. May mediate the reciprocal stimulatory effect of splicing on transcriptional elongation (By similarity). Component of a complex which is at least composed of HTATSF1/Tat-SF1, the P-TEFb complex components CDK9 and CCNT1, RNA polymerase II, SUPT5H, and NCL/nucleolin. Interacts with GTF2F2/RAP30 and POLR2A. Interacts with TCERG1/CA150. Interacts with SF3A2/SAP62 and the spliceosomal U small nuclear ribonucleoproteins (snRNPs) (By similarity). The RRM domains mediate interaction with U snRNPs. Belongs to the HTATSF1 family.