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

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.