The MSL complex levels are critical for its correct targeting to the chromosomes in Drosophila melanogaster

In Drosophila, dosage compensation requires assembly of the Male Specific Lethal (MSL) protein complex for doubling transcription of most X-linked genes in males. The recognition of the X chromosome by the MSL complex has been suggested to include initial assembly at approximately 35 chromatin entry sites and subsequent spreading of mature complexes in cis to numerous additional sites along the chromosome. To understand this process further we examined MSL patterns in a range of wild-type and mutant backgrounds producing different amounts of MSL components. Our data support a model in which MSL complex binding to the X is directed by a hierarchy of target sites that display different affinities for the MSL proteins. Chromatin entry sites differ in their ability to provide local intensive binding of complexes to adjacent regions, and need high MSL complex titers to achieve this. We also mapped a set of definite autosomal regions (approximately 70) competent to associate with the functional MSL complex in wild-type males. Overexpression of both MSL1 and MSL2 stabilizes this binding and results in inappropriate MSL binding to the chromocenter and the 4th chromosome. Thus, wild-type MSL complex titers are critical for correct targeting to the X chromosome.