Two regions in yeast are currently being studied, the his 3 locus and the galactose induced cluster region. A large number of deletions have been isolated that affect the his 3 gene expression. Those deletions which have endpoints outside of the region coding for the RNA and yet block expression of this his 3 gene and the production of RNA, have been identified as promoter mutations. The sequence of the region covered by these deletion endpoints has been determined. The exact endpoints of the deletions are currently being mapped by sequencing techniques. Restriction sites are being introduced at these deletion endpoints by fusion to synthetic DNAs. This gene that does not have a promoter and cannot be expressed in the yeast cell will be fused to other regions of the yeast genome. This should then serve as an assay for promoter function in yeast. The other region in yeast which is being studied is a region that is induced on growth in galactose. This region is likely the gal 1, 7 and 10 gene cluster. It codes for three major stable RNAs. The exact endpoints of these RNAs are currently being mapped. There is also evidence that precursor RNAs are made in this region. The exact functional relationship of these very large RNAs to more abundant small RNAs is being studied by making deletions with endpoints within the large transcript that do not correspond to the small RNAs. These alterations are then introduced into the yeast by yeast transformation with the newly developed vector systems that initially autonomously replicate and then integrate into the corresponding homologous locus in the yeast genome. The promoter regions for the galactose gene clusters are also being identified by fusing small sections of DNA to the his 3 gene without its promoter. The proper fusions should allow his 3 gene expression only in the presence of galactose.
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