9526802 Tartof Heterochromatin comprises about thirty percent of the Drosophila chromosome. Position effect variegation (PEV) results when a euchromatic gene has been placed adjacent to a block of centromeric heterochromatin. Such PEV can be modified by dominant genes known as Enhancers of variegation and Suppresors of variegation. Those genes can be divided into two classes, I and II, depending upon their response to gene dosage. A putative clone of a class II gene will be positively identified and characterized by transformation and genetic analysis. If it is shown to be a class II gene, experiments can be designed to attempt to understand the regulating role of heterochromatin on gene expression. %%% The position of a gene on a chromosome can determine its ability to be expressed. Most of the genes in a eukaryotic chromosome are found in loosely packed regions of DNA, called euchromatin, and if they are moved close to, or into, regions of densely packed DNA, called heterochromatin, either express at a low level in the first case or not at all in the second case. When a gene is inserted into a chromosome the level of expression can vary between cells and result in what is called position effect variagation. It is postulated that not only is the position of the gene a determining factor in its level of expression but that other genes called suppressors and enhancers can also act to modify the level of expression. A putative suppressor gene has been cloned and if characterization of that gene supports its identity as such, experiments can be designed to attempt to understand the regulating role of the heterochromatin as well as the gene itself on other genes. ***
