POLYPURINE POLYPYRIMIDINE SEQUENCES AS CIS-ACTING TRANSCRIPTIONAL REGULATORS/

Genome sequence information has generated increasing evidence for the claim that repetitive DNA sequences present within and around genes co uld play a important role in the regulation of gene expression. Polypu rine/polypyrimidine sequences [poly(Pu/Py)] have been observed in the vicinity of promoters and within the transcribed regions of many genes . To understand whether such sequences influence the level of gene exp ression, we constructed several prokaryotic and eukaryotic expression vectors incorporating poly(Pu/Py) repeats both within and upstream of a reporter gene, lacZ (encoding beta-galactosidase), and studied its e xpression in vivo. We find that, in contrast to the situation in Esche richia coli, the presence of poly(Pu/Py) sequences within the gene doe s not significantly inhibit gene expression in mammalian cells. On the other hand, the presence of such sequences upstream of lacZ leads to a several-fold reduction of gene expression in mammalian cells. Simila r down-regulation was observed when a structural cassette containing p oly(Pu/Py) sequences upstream of lacZ was integrated into yeast chromo some V. Sequence analysis of the nine totally sequenced yeast chromoso mes shows that a large number of such sequences occur upstream of ORFs . On the basis of our experimental results and DNA sequence analysis, we propose that these sequences can function as cis-acting transcripti onal regulators.