Heterochromatic silencing of Drosophila heat shock genes acts at the levelof promoter potentiation

In a variety of organisms, genes placed near heterochromatin are transcript ionally silenced. In order to understand the molecular mechanisms responsib le for this block in transcription, high resolution in vivo chromatin struc ture analysis was performed on two heat shock genes, hsp26 and hsp70, These genes normally reside in euchromatin where GAGA factor and RNA Pol II are present on the promoter prior to heat shock induction. P-element transforma tion experiments led to the identification of stocks in which these two gen es were inserted within heterochromatin of the chromosome 4 telomeric regio n. These transgenes exhibit silencing that is partially suppressed by mutat ions in the gene encoding HP1. Micrococcal nuclease analysis revealed that the heterochromatic transgenes were packaged in a more regular nucleosome a rray than when located in euchromatin. High resolution DNase I analysis dem onstrated that GAGA factor and TFIID were not associated with these promote rs in heterochromatin; potassium permanganate experiments showed a loss of Pol II association. Taken together, these data suggest that occlusion of tr ansacting factors from their cis-acting regulatory elements leading to a bl ock in promoter potentiation is a mechanism for heterochromatin gene silenc ing.