The SPT4-SPT5 complex mediates both activation and inhibition of transcription elongation; and plays a role in pre- mRNA processing. The 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. Structural and functional component of the centromeric and heterochromatic loci linking chromatin structure with kinetochore function and gene silencing. {ECO:0000269|PubMed:12554661; ECO:0000269|PubMed:12556496; ECO:0000269|PubMed:15057281; ECO:0000269|PubMed:15780939; ECO:0000269|PubMed:16172632; ECO:0000269|PubMed:8483459; ECO:0000269|PubMed:9450930}.