THE DOSAGE COMPENSATION REGULATORS MLE, MSL-1 AND MSL-2 ARE INTERDEPENDENT SINCE EARLY EMBRYOGENESIS IN DROSOPHILA

We have analyzed the expression pattern and localization of MLE, MSL-1 , MSL-2, and histone H4Ac16 during embryogenesis to determine when msl -dependent dosage compensation begins. Maternal MSL-1 and MLE are pres ent in both sexes at fertilization. MSL-2 lacks a maternal component, and male-specific zygotic expression is detectable at the end of blast oderm. During germ band extension, MSL-1, MSL-2, MLE, and histone H4Ac 16 display coincident sub-nuclear localization in male embryos. In emb ryos lacking one of the MSL proteins, the sub-nuclear localization of the other MSLs and of histone H4Ac16 is not detected. We conclude that the MSL proteins associate with the X chromosome and are interdepende nt since early embryogenesis.