Previous characterization of the Saccharomyces cerevisiae Spt4, Spt5, and Spt6 
proteins suggested that these proteins act as transcription factors that modify 
chromatin structure. In this work, we report new genetic and biochemical studies 
of Spt4, Spt5, and Spt6 that reveal a role for these factors in transcription 
elongation. We have isolated conditional mutations in SPT5 that can be 
suppressed in an allele-specific manner by mutations in the two largest subunits 
of RNA polymerase II (Pol II). Strikingly, one of these RNA Pol II mutants is 
defective for transcription elongation and the others cause phenotypes 
consistent with an elongation defect. In addition, we show that spt4, spt5, and 
spt6 mutants themselves have phenotypes suggesting defects in transcription 
elongation in vivo. Consistent with these findings, we show that Spt5 is 
physically associated with RNA Pol II in vivo, and have identified a region of 
sequence similarity between Spt5 and NusG, an Escherichia coli transcription 
elongation factor that binds directly to RNA polymerase. Finally, we show that 
Spt4 and Spt5 are tightly associated in a complex that does not contain Spt6. 
These results, taken together with the biochemical identification of a human 
Spt4-Spt5 complex as a transcription elongation factor (Wada et al. 1998), 
provide strong evidence that these factors are important for transcription 
elongation in vivo.