Abstract
Genes in multicellular organisms are expressed as part of a developmental program that is largely dependent on self-perpetuating higher-order chromatin states. The mechanism of establishing and maintaining these epigenetic events is well studied in Drosophila. The first known example of an epigenetic effect was that of (PEV) in Drosophila, which has been shown to be due to gene silencing via heterochromatin formation. We are investigating a process similar to Position Effect Variegation (PEV) using a mini-w transgene, called Pci, inserted in the upstream regulatory region of ci. The mini-white + transgene in Pci is expressed throughout the adult eye; however, when other P or KP elements are present, a variegated eye phenotype results indicating random w + silencing during development. This P element dependent silencing (PDS) can be modified by the haplo-suppressors/triplo-enhancers, Su(var)205 and Su(var)3–7, indicating that these heterochromatic modifiers also act dose dependently in PDS. Here we use a spontaneous derivative mutation of Pci called PciE1 (E1) that variegates like PDS in the absence of P elements, presumably due to an adjacent gypsy element insertion, to screen for second-site modifier mutations that enhance variable silencing of white + in E1. We isolated 7 mutations in CG8878, an essential gene, that enhance the E1 variegated phenotype. CG8878, a previously uncharacterized gene, potentially encodes a serine/threonine kinase whose closest Drosophila paralogue, ballchen (nhk-1), phosphorylates histones. These mutant alleles enhance both PDS at E1 and Position Effect Variegation (PEV) at wm4, indicating a previously unknown common silencing mechanism between the two.
Highlights
In Drosophila melanogaster, expression of the white+ gene (w+) is cell autonomous, and necessary for the import of pigment precursors for normal colour in the adult eye
The Pci transgene is sensitive to the presence of P elements in the genome in a phenomenon called P element dependent silencing (PDS), which is phenotypically similar to heterochromatic position effect variegation
We describe here the isolation and characterization of seven mutants, that enhance both PDS and heterochromatic position effect variegation (hPEV) at wm4
Summary
In Drosophila melanogaster, expression of the white+ gene (w+) is cell autonomous, and necessary for the import of pigment precursors for normal colour in the adult eye. The absence of pigment results in a white-eyed phenotype that can be rescued with w+ containing P-element transgenes. In some insertion locations, expression of w+ is sensitive to the local chromatin environment, such as adjacent heterochromatin. In flies lacking P elements (M strains) the w+ transgene is expressed in a uniform manner (even red eye phenotype). In flies containing P elements (P strains) or KP s (derivative elements capable of mimicking some of the characteristics of P strains such as modifying P- repressor sensitive alleles, but not enabling P element transposition) variegated expression occurs resulting in a mosaic expression of white ommatidia on a red background in the eye [3]. PDS occurs when other w+ transgenes are inserted near this location [2]
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