Relationships between chromosome structure and long distance regulation of gene expression: A study of cis and trans modifiers of terminal deficiency-associated position effect variegation in a Drosophila melanogastor minichromosome

Genetic and cytological evidence indicate that chromosome structure and the organization of chromosomes within the interphase nucleus play major roles in the long-distance regulation of gene expression, and in chromosome inheritance. I have used the position effect variegation (PEV) associated with derivatives of Drosophila minichromosome Dp1187 to study cis and trans effects of chromosome structure and organization on gene expression. PEV is the clonal inactivation of a euchromatic gene that is located within or near heterochromatin. Terminal deletions of Dp1187 increase the PEV of the yellow+ gene located on it; this terminal deficiency-associated PEV (TDA-PEV) is suppressed by the presence of a second minichromosome, a novel phenomenon termed "trans -suppression". I examined the chromosomal elements responsible for trans-suppression using a series of minichromosomes with molecularly-characterized deletions and inversions. Full trans-suppression requires substantial chromosome homology, suggesting that chromosome pairing plays a key role in trans-suppression. These data indicate TDA-PEV and trans-suppression may reflect changes in nuclear positioning of the chromosomes and the gene, and/or distribution and activity of telomere binding proteins. My preliminary studies using fluorescence in situ hybridization (FISH) indicate the TDA-PEV and trans -suppressing minichromosomes pair in interphase nuclei, supplying corroborative evidence that trans-suppression involves chromosome pairing.; The proteins and mechanisms responsible for nuclear positioning, somatic pairing, and protecting and packaging chromosome ends in Drosophila have not been elucidated. To identify such proteins involved in these basic chromosomal functions, and to test models for long-distance regulation of gene expression, I screened for mutations that affect TDA-PEV and trans-suppression. Seventy-one mutations were identified. By mapping of these mutations, and characterizing them for their ability to affect other variegating alleles, I identified ten which specifically affect TDA-PEV. The remaining 61 mutations affect multiple types of PEV, suggesting they encode proteins with general roles in chromosome biology. Indeed, a high proportion of these mutations have a dominant effect on the transmission of a "centromere challenged" minichromosome, suggesting they affect genes critical in chromosome inheritance. My data indicate heterochromatin is a crucial component of normal chromosome function, and of the mechanisms undertaken by chromosome structure and nuclear organization in regulating gene expression and chromosome inheritance.