Poly purine.pyrimidine sequences upstream of the beta-galactosidase gene affect gene expression in Saccharomyces cerevisiae

A poly purine.pyrimidine sequence with the potential to adopt an intramolecular triplex DNA structure was designed. The sequence was inserted within a nucleosome positioned upstream of the β-galactosidase gene in yeast, Saccharomyces cerevisiae, between the cycl promoter and gal 10Upstream Activating Sequences (UASg). Upon derepression with galactose, β-galactosidase gene expression is reduced 12-fold in cells carrying single copy poly purine.pyrimidine sequences. This reduction in expression is correlated with reduced transcription. Furthermore, we show that plasmids carrying a poly purine.pyrimidine sequence are not specifically lost from yeast cells.We propose that a poly purine.pyrimidine sequence upstream of a gene affects transcription. Plasmids carrying this sequence are not specifically lost from cells and thus no additional effort is needed for the replication of these sequences in eukaryotic cells.Stretches of poly purine.pyrimidine (poly pur.pyr) sequences are overrepresented in the eukaryotic genome [1,2] and often positioned upstream of genes [3,4]. Many eukaryotic genes contain a poly pur.pyr box upstream of the coding region [5]. Transcription factors may interact specifically with those regions as occurs upstream of the Drosophila hsp26 gene [6,7] where transcription is enhanced during a heat shock.Under torsional stress poly pur.pyr sequences have the potential to adopt an intramolecular triplex formation as HY-3 and H-Y5 [8-10]. A proposed triplex model fits with the regular poly pur.pyr tract with mirror symmetry [11] around a loop sequence. Shimizu et al.[10] have shown that base composition in loop sequences affects intramolecular triplex formation while orientation of the insert in supercoiled plasmid has no effect on isomerisation of the two types of triplex DNA. Nonmirror symmetric poly pur.pyr sequences can also adopt an intramolecular triplex structure [12].The in vivo existence of poly pur.pyr sequences as an intramolecular triplex has bee