Dosage compensation in flies: Function and regulation of the roX RNAs

The roX1 and roX2 genes of Drosophila melanogaster produce male-specific noncoding RNAs that colocalize with the Male-Specific Lethal (MSL) protein complex. The MSL complex mediates the transcriptional upregulation of the male X chromosome, contributing to the equalization of X-linked gene expression between male and female flies in a process called dosage compensation. The roX RNAs share features that suggest they are involved in this essential process. Nevertheless, mutations in roX1 present no phenotype, suggesting that roX1 and roX2 might be redundant. Intriguingly, they share almost no sequence similarities.; In this thesis I present results that led to the verification that the roX1 and roX2 genes produce redundant male-specific lethal transcripts required for dosage compensation. P elements in the roX2 region were identified and flies lacking roX2 were generated by imprecise excision. This gene is also non-essential, but simultaneous mutation of roX1 and roX2 causes male-specific lethality and misslocalization of MSL complexes. Ectopic activation of dosage compensation in females is lethal, but elimination of both roX transcripts restores viability, indicating that they play an essential role in this process.; The regulation of the roX genes was also studied. Only one MSL subunit, the MSL2 protein, appears responsible for driving the transcription of the roX genes only in males. Although all the MSL proteins are required for roX posttranscriptional stabilization, no other subunit is necessary for roX male-specific transcription. This novel activity of MSL2 does not rely on the RING Finger domain, a region essential for the role of this protein in dosage compensation. The regulation of roX by MSL2 and the essentiality of the roX transcripts for dosage compensation suggest a mechanism by which an optimal MSL/ roX ratio that is favorable for the normal localization of the complex to the X chromosome may be maintained.