Heterochromatin is both necessary for the expression of heterochromatic genes and inhibitory for the expression of euchromatic genes. These two properties of heterochromatin have been elucidated from the study of chromosome rearrangements that induce position effect variegation (PEV) in Drosophila melanogaster. Novel euchromatin-heterochromatin junctions can affect the expression of euchromatic and heterochromatic genes located several megabases away, distinguishing higher order chromatin structure from most other regulatory mechanisms. Studies of PEV promise insights into the basis for heterochromatin formation and the role of higher order chromatin and chromosome structure in gene regulation. We evaluate the models and experimental data that address the mechanisms of PEV in different cell types, the potential functions of modifiers of PEV, and the relationship of PEV to other phenomena associated with variegated gene expression in Drosophila.

The importance of a gene's natural chromatin environment for its normal expression is poignantly illustrated when a change in chromosome position results in variable gene repression, such as is observed in position effect variegation (PEV) when the Drosophila melanogaster white (w) gene is juxtaposed with heterochromatin. The Enhancer of variegation 3-9 ½E(var)3-9 gene was one of over a hundred loci identified in screens for mutations that dominantly modify PEV. Haploinsufficiency for E(var)3-9 enhances w m4 variegation, as would be expected from increased heterochromatin formation. To clarify the role of E(var)3-9 in chromosome structure, the gene has been cloned and its mutant alleles characterized. The involvement of E(var)3-9 in structure determination was supported by its reciprocal effects on euchromatic and heterochromatic PEV; E(var)3-9 mutations increased expression of a variegating heterochromatic gene in two tissue types. E(var)3-9 mutations also had a recessive phenotype, maternal effect lethality, which implicated E(var)3-9 function in an essential process during embryogenesis. Both phenotypes of E(var)3-9 mutations were consistent with its proposed function in promoting normal chromosome structure. The cloning of E(var)3-9 by classical genetic methods revealed that it encodes a protein with multiple zinc fingers, but otherwise novel sequence. E UKARYOTIC genomes exhibit differential packaging, and distinctions in chromosome structure are associated with differing genetic properties, such as transcriptional activity, centromere function, chromosome pairing, and replication timing. Position effect variegation (PEV) in Drosophila melanogaster has long been a model system for studying determinants of chromosome structure (reviewed by Weiler and Wakimoto 1995;Schotta et al. 2003). PEV results from chromosome rearrangements with breakpoints in cytologically distinct chromosome regions, the densely staining heterochromatin, which is rich in repetitive sequences but gene-poor, and the lightly staining euchromatin characterized by a high density of genic DNA. Euchromatic genes in the vicinity of the breakpoint are subject to inactivation when brought into proximity of heterochromatin, although this repression exhibits cell-to-cell variability. The early cytological studies of polytene chromosomes from individuals bearing variegation-inducing rearrangements first indicated that mosaic inactivation was due to the variable spreading of a heterochromatic chromatin structure into the formerly euchromatic region and have since been supported by molecular assays (Hayashi et al. 1990;Wallrath and Elgin 1995;Sun et al. 2000). More recently, a role for histone modifications and small RNAs in heterochromatin formation and heterochromatin-mediated silencing has been established (Grewal and Rice 2004). However, many questions remain about the mechanism(s) of heterochromatin formation, silencing and spreading, the role of transinteractions, and the gene products that regulate the euchromatic and heterochromatic s...

The D1 protein is a high mobility group A (HMGA)-like nonhistone chromosomal protein with primary localization to certain AT-rich satellite DNA sequences within heterochromatin. The binding of D1 to euchromatic sequences is less studied and the functional significance of its chromosomal associations is unclear. By taking advantage of existing P-insertion alleles of the D1 gene, I generated D1 null mutations to investigate the phenotypic effect of loss of the D1 gene. In contrast to a previous report, I determined that the D1 gene is not essential for viability of Drosophila melanogaster, and moreover, that loss of D1 has no obvious phenotypic effects. My tests for an effect of D1 mutations on PEV revealed that it is not a suppressor of variegation, as concluded by other investigators. In fact, the consequence of loss of D1 on one of six variegating rearrangements tested, T(2;3)Sb V , was dominant enhancement of PEV, suggesting a role for the protein in euchromatic chromatin structure and/or transcription. A study of D1 protein sequence conservation highlighted features shared with mammalian HMGA proteins, which function as architectural transcription factors.