The SPT4, SPT5 and SPT6 genes of Saccharomyces cerevisiae were identified 
originally by mutations that suppress delta insertion mutations at HIS4 and 
LYS2. Subsequent analysis has demonstrated that spt4, spt5 and spt6 mutations 
confer similar pleiotropic phenotypes. They suppress delta insertion mutations 
by altering transcription and are believed to be required for normal 
transcription of several other loci. We have now analyzed interactions between 
SPT4, SPT5 and SPT6. First, the combination of mutations in any two of these 
three genes causes lethality in haploids. Second, some recessive mutations in 
different members of this set fail to complement each other. Third, mutations in 
all three genes alter transcription in similar ways. Finally, the results of 
coimmunoprecipitation experiments demonstrate that at least the SPT5 and SPT6 
proteins interact physically. Taken together, these genetic and biochemical 
results indicate that SPT4, SPT5 and SPT6 function together in a transcriptional 
process that is essential for viability in yeast.