Three Genes Are Required for trans-Activation of Ty Transcription in Yeast

Mutations in the SPT3 gene were isolated as one class of suppressors of Ty and solo delta insertion mutations in Saccharomyces cerevisiae. Previous work has shown that null mutations in SPT3 abolish the normal Ty delta-delta transcript; instead, a transcript that initiates 800 bases farther downstream is made, suggesting that SPT3 is required for transcription initiation in delta sequences. We have selected for new spt mutations and have screened for those with the unique suppression pattern of spt3 mutations with respect to two insertion mutations. Our selection and screen has identified two additional genes, SPT7 and SPT8, that are also required for transcription initiation in delta sequences. We show that mutations in SPT7 or SPT8 result in the same alteration of Ty transcription as do mutations in SPT3. In addition, mutations in all three genes cause a sporulation defect. By assay of a Ty-lacZ fusion we have shown that spt3, spt7 and spt8 mutations reduce transcription from a delta sequence by 10-25-fold. Finally, we show that SPT3 mRNA levels are unaffected in either spt7 or spt8 mutants, suggesting that these two genes do not regulate transcription of SPT3.