Component of the DRB sensitivity-inducing factor complex (DSIF complex), which regulates mRNA processing and transcription elongation by RNA polymerase II. DSIF positively regulates mRNA capping by stimulating the mRNA guanylyltransferase activity of RNGTT/CAP1A. DSIF also acts cooperatively with the negative elongation factor complex (NELF complex) to enhance transcriptional pausing at sites proximal to the promoter. Transcriptional pausing may facilitate the assembly of an elongation competent RNA polymerase II complex. DSIF and NELF promote pausing by inhibition of the transcription elongation factor TFIIS/S-II. TFIIS/S-II binds to RNA polymerase II at transcription pause sites and stimulates the weak intrinsic nuclease activity of the enzyme. Cleavage of blocked transcripts by RNA polymerase II promotes the resumption of transcription from the new 3' terminus and may allow repeated attempts at transcription through natural pause sites (By similarity). Interacts with SUPT5H to form DSIF. DSIF interacts with the positive transcription elongation factor b complex (P-TEFb complex), which is composed of CDK9 and cyclin-T (CCNT1 or CCNT2). DSIF interacts with RNA polymerase II, and this interaction is reduced by phosphorylation of the C-terminal domain (CTD) of POLR2A by P-TEFb. DSIF also interacts with the NELF complex, which is composed of WHSC2/NELFA, COBRA1/NELFB, TH1L/NELFD and RDBP/NELFE, and this interaction occurs following prior binding of DSIF to RNA polymerase II. DSIF also interacts with HRMT1L2/PRMT1, HTATSF1/TATSF1, RNGTT/CAP1A, SKB1/PRMT5, SUPT6H, and can interact with PIN1 (By similarity). Expressed in brain, heart and liver. Belongs to the SPT4 family.