The spt4-spt5 complex mediates both activation and inhibition of transcription elongation, and plays a role in pre-mRNA processing. This complex seems to be important for the stability of the RNA polymerase II elongation machinery on the chromatin template but not for the inherent ability of this machinery to translocate down the gene (By similarity). Component of the spt4-spt5 complex. Interacts with RNA polymerase II (By similarity). Belongs to the SPT5 family.