Figure 1. (a) Schematic illustration of white variegation in a Drosophila X-chromosome inversion. The white locus (w⁺) is located in the distal euchromatin (thin line) of the wild-type X-chromosome. It provides a function essential to normal pigmentation of the fly's eye. An inversion induced by X-rays, with one breakpoint in the pericentric heterochromatin and one breakpoint close to the white gene, places the white gene within several kilobases of the pericentric heterochromatin (thick line). Heterochromatic proteins (geometric forms), which normally assemble only in heterochromatic regions, may now spread across the breakpoint in some cells, potentially packaging the gene in a heterochromatic form, resulting in silencing (white facet) in those cells. (b) Given a variegating phenotype, second-site mutations that result in a loss of silencing can be recovered; such mutations (suppressors of position effect) usually identify genes that encode proteins that contribute to the heterochromatin structure. Similarly, second site mutations that result in increased silencing (enhancers of position effect) often identify genes that contribute to the active state.

Figure 2. Packaging of yeast telomeres. Binding of RAP1 may be a key nucleating event in the assembly of these complexes. Complexes of Sir3 and Sir4 proteins bind to the hypoacetylated, hypomethylated histones, extending along the DNA; Sir2 is also critical for stable maintenance of the silenced state.

Figure 3. Distribution of nonhistone chromosomal proteins in the polytene chromosomes of Drosophila melanogaster shown using immunofluorescence. Top: phase-contrast picture. Note that the extended, euchromatic arms are fused in a common heterochromatic chromocentre. Middle: distribution of GAGA factor (green), a protein that plays a role in gene activation. Bottom: distribution of HP1 (red), a protein primarily associated with heterochromatin that plays a role in gene silencing. Photo courtesy of C. Craig.