Epigenetic spreading of the Drosophila dosage compensation complex from roX RNA genes into flanking chromatin

The multisubunit MSL dosage compensation complex binds to hundreds of sites along the Drosophila single male X chromosome, mediating its hypertranscri ption, The male X chromosome is also coated with noncoding roX RNAs. When e ither msl3, mle, or mof is mutant, a partial MSL complex is bound at only s imilar to 35 unusual sites distributed along the X, We show that two of the se sites are the roX1 and roX2 genes and postulate that one of their functi ons is to provide entry sites for the MSL complex to recognize the X chromo some. The roX1 gene provides a nucleation site for extensive spreading of t he MSL complex into flanking chromatin even when moved to an autosome. The spreading can occur in cis or in trans between paired homologs. We present a model for how the dosage compensation complex recognizes X chromatin.