9305846 Robbins Heterochromatin is a substantial fraction of every eukaryotic genome and has a number or remarkable characteristics. Nevertheless, few heterochromatic functions have been identified. Among those few are the ribosomal RNA genes (rDNA)and Rex (Ribosomal exchange), a maternal-effect cominant that induces rDNA recombination during early mitotic divisions in embryos produced by Rex females. Rex has now been shown to be a repeated element located within the rDNA array. Deletion of rDNA repeats at the sites of Rex induced exchange suggests that chromosome breakage is involved. An endonuclease produced by an active variant of one of the insertion sequences found in all rDNA arrays could be the basis of active variant of one of the insertion sequences found in all rDNA arrays could be the basis of Rex. Analysis of this system will continue in three areas: 1) Molecular and genetic analysis of the nature and action of Rex, 2) Rapid mapping of rDNA arrays by use of Rex-induced recombination, 3) Analysis of the properties of recombinant rDNA arrays. %%% Roughly thirty percent of the genome of the fruit fly is constitutive heterochromatin and yet little is known about either its structure or function. Rex can provide a tool for learning more about these chromosomal regions in the fruit fly, which knowledge can be then extrapolated to other eukaryotic organisms in which these type of experiments cannot be done. ***
