The Homopurine-Homopyrimidine Dnana Sequence Flanking The Thyroglobulin Gene Does Not Behave As A Transcriptional Diode In Living Cells

Homopurine-homopyrimidine DNA sequences are found at higher than expected frequency around gene promoters in mammalian genomes. They were reported to act as a transcriptional diode in in vitro assays, as only the homopurine DNA strand was transcribed. We show here that the homopurine-homopyrimidine upstream element of the thyroglobulin gene promoter is able to reduce transcription initiated from different promoters, both on transiently transfected plasmids or when integrated into the genome of rat thyroid cells. In living cells, this homopurine-homopyrimidine sequence inhibits transcription of both DNA strands.