Stimulates transcription elongation. Heterodimer composed of Spt4 and Spt5. Interacts with RNA polymerase (RNAP) independently of nucleic acids. Forms a homodimer in solution. Composed of only a NusG N-terminal (NGN) domain and a KOW domain, similar to bacterial NusG. The NGN domain is the effector domain of the complex that mediates the interaction with RNAP. The NGN domain closes the RNAP active center cleft to lock nucleic acids and render the elongation complex stable and processive. The KOW domain may interact with RNA and/or other factors. Belongs to the archaeal Spt5 family.