May function as a component of the DRB sensitivity-inducing factor complex (DSIF complex), which regulates transcription elongation by RNA polymerase II. DSIF may enhance transcriptional pausing at sites proximal to the promoter, which may in turn facilitate the assembly of an elongation competent RNA polymerase II complex. Interacts with spt-5 to form DSIF. DSIF interacts with RNA polymerase II and with the positive transcription elongation factor b complex (P-TEFb complex), which is composed of cdk-9 and cyclin-T (cit-1.1 or cit-1.2) (By similarity). Belongs to the SPT4 family.