Component of the DRB sensitivity-inducing factor complex (DSIF complex), which regulates mRNA processing and transcription elongation by RNA polymerase II (PubMed:10075709, PubMed:10199401, PubMed:10421630, PubMed:10757782, PubMed:10912001, PubMed:11112772, PubMed:11553615, PubMed:12653964, PubMed:12718890, PubMed:15136722, PubMed:15380072, PubMed:9450929, PubMed:9857195). DSIF positively regulates mRNA capping by stimulating the mRNA guanylyltransferase activity of RNGTT/CAP1A (PubMed:10075709, PubMed:10421630, PubMed:10757782, PubMed:10912001, PubMed:11112772, PubMed:11553615, PubMed:12653964, PubMed:12718890, PubMed:15136722, PubMed:15380072, PubMed:9450929, PubMed:9857195). DSIF also acts cooperatively with the negative elongation factor complex (NELF complex) to enhance transcriptional pausing at sites proximal to the promoter (PubMed:10075709, PubMed:10199401, PubMed:10757782, PubMed:10912001, PubMed:11112772, PubMed:11553615, PubMed:12653964, PubMed:12718890, PubMed:15136722, PubMed:15380072, PubMed:9450929, PubMed:9857195). Transcriptional pausing may facilitate the assembly of an elongation competent RNA polymerase II complex (PubMed:10075709, PubMed:10199401, PubMed:10421630, PubMed:10757782, PubMed:10912001, PubMed:11112772, PubMed:11553615, PubMed:12653964, PubMed:12718890, PubMed:15136722, PubMed:15380072, PubMed:9450929, PubMed:9857195). DSIF and NELF promote pausing by inhibition of the transcription elongation factor TFIIS/S-II (PubMed:16214896). TFIIS/S-II binds to RNA polymerase II at transcription pause sites and stimulates the weak intrinsic nuclease activity of the enzyme (PubMed:16214896). 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 (PubMed:16214896). Following phosphorylation by CDK9, DSIF can also positively regulate transcriptional elongation (PubMed:16427012). Required for the efficient activation of transcriptional elongation by the HIV-1 nuclear transcriptional activator, Tat (PubMed:10393184, PubMed:10454543, PubMed:11809800, PubMed:9514752). DSIF acts to suppress transcriptional pausing in transcripts derived from the HIV-1 LTR and blocks premature release of HIV-1 transcripts at terminator sequences (PubMed:11112772, PubMed:14701750). {ECO:0000269|PubMed:10075709, ECO:0000269|PubMed:10199401, ECO:0000269|PubMed:10393184, ECO:0000269|PubMed:10421630, ECO:0000269|PubMed:10454543, ECO:0000269|PubMed:10757782, ECO:0000269|PubMed:10912001, ECO:0000269|PubMed:11112772, ECO:0000269|PubMed:11553615, ECO:0000269|PubMed:11809800, ECO:0000269|PubMed:12653964, ECO:0000269|PubMed:12718890, ECO:0000269|PubMed:14701750, ECO:0000269|PubMed:15136722, ECO:0000269|PubMed:15380072, ECO:0000269|PubMed:16214896, ECO:0000269|PubMed:16427012, ECO:0000269|PubMed:9450929, ECO:0000269|PubMed:9514752, ECO:0000269|PubMed:9857195}. This is the function of SUPT5H (SPT5 homolog, DSIF elongation factor subunit, Ensembl gene identifier ENSG00000196235).