The severity of roX1 mutations is predicted by MSL localization on the X chromosome

Abstract

Dosage compensation equalizes the expression of sex-linked genes between males and females. Most genes on the X chromosome of male Drosophila are transcribed at an increased level, contributing to compensation. The roX1 and roX2 genes produce non-coding transcripts that localize along the X-chromosome of male flies. Although lacking sequence similarity, they are necessary but redundant components of a system that up-regulates gene expression. Simultaneous mutation of both roX genes disrupts the X-limited distribution of proteins that modify chromatin to enhance gene expression. We have generated and characterized loss of function roX1 alleles that display a continuum of activity. Those that support intermediate male survival have strikingly reduced RNA accumulation, while alleles with minor contributions to male viability typically lack detectable transcript accumulation. Severely mutated roX1 alleles retain some ability to direct modifying proteins to the X chromosome. This ability predicts the level of male survival that each allele supports. This points to a peripheral or transient role for roX in the RNA and protein complex that binds to and regulates the X chromosome.

roXRNAs Are Required for Increased Expression of X-Linked Genes inDrosophila melanogasterMales

“…The roX1 ex6 , roX1 SMC17A , and roX2 mutations have been previously described (Meller et al 1997;Meller and Rattner 2002;Deng et al 2005).…”

“…By contrast, simultaneous mutation of both roX genes prevents the exclusive binding of MSL proteins to the X chromosome. Reduced levels of MSL proteins are retained at some sites on the X chromosome, but they are now detected at a number of ectopic autosomal sites (Meller and Rattner 2002;Deng et al 2005). Some of these sites are puffed, a chromatin state usually associated with active transcription.…”

A male‐specific effect of dominant‐negative Fos

“…We investigated genetic interactions between the msl and mle mutations, and vg-Fbz, which shows a male-limited wing phenotype. We also tested interactions with an X chromosome containing loss of function for both the rox1 and roX2 redundant loci (Meller and Rattner, 2002;Deng et al, 2005).…”

“…However, significant enhancement of the vg-Fbz wing phenotype (77.4%) was apparent after coexpression of msl-3 RNAi, corroborating the msl allele results (Table 1). Finally, we examined if vg-Fbz interacted with an X chromosome deficient for roX1 and roX2 functions: roX1 ex21A Df(1)52 (Deng et al, 2005). In this experiment, the lethality associated with roX2 deficiency (Df(1)52) was covered with a rescuing transgene (wϩ4⌬4.3) that does not contain roX2 (Meller and Rattner, 2002).…”

“…Therefore, one hypothesis to explain the male phenotypes from Fos bZip expression involves interference with the assembly of the complex through effects on Msl1 or Msl2. To test this possibility, we determined if overexpression of heat shock-regulated wild-type Msl1, Msl2, or the combination Msl 1ϩMsl2 (Deng et al, 2005) could rescue the Fos bZip wing phenotype. After a daily heat shock regimen, we observed partial, but significant rescue of the vg-Fbz phenotype with expression of the Msl1 and Msl1ϩMsl2 constructs (Table 2), indicating an effect of Msl1 overexpression.…”