The roX genes encode redundant male‐specific lethal transcripts required for targeting of the MSL complex

The roX1 and roX2 genes of Drosophila produce male‐specific non‐coding RNAs that co‐localize with the Male‐Specific Lethal (MSL) protein complex. This complex mediates up‐regulation of the male X chromo some by increasing histone H4 acetylation, thus contributing to the equalization of X‐linked gene expression between the sexes. Both roX genes overlap two of ∼35 chromatin entry sites, DNA sequences proposed to act in cis to direct the MSL complex to the X chromosome. Although dosage compensation is essential in males, an intact roX1 gene is not required by either sex. We have generated flies lacking roX2 and find that this gene is also non‐essential. However, simultaneous removal of both roX RNAs causes a striking male‐specific reduction in viability accompanied by relocation of the MSL proteins and acetylated histone H4 from the X chromosome to autosomal sites and heterochromatin. Males can be rescued by roX cDNAs from autosomal transgenes, demonstrating the genetic separation of the chromatin entry and RNA‐encoding functions. Therefore, the roX1 and roX2 genes produce redundant, male‐specific lethal transcripts required for targeting the MSL complex.