DOSAGE COMPENSATION IN DROSOPHILA - THE X-CHROMOSOME BINDING OF MSL-1AND MSL-2 IN FEMALE EMBRYOS IS PREVENTED BY THE EARLY EXPRESSION OF THE SXL GENE

In wildtype males, binding of the MSL-1 gene product to the X chromoso me is first seen at the cellular blastoderm stage (stage 5). MSL-2 is associated with the X chromosome in male embryos at a later stage, but the difference in apparent binding time between these two proteins is probably due to a difference in the sensitivity of their respective a ntisera. Early binding of MSL-1 is never seen in wildtype female embry os, and we have determined that this inhibition is mediated by the SXL product made by the activation of the early Sri promoter. Once it is allowed to occur, the early X chromosome association of the MSLs is re latively stable, persisting in some cases through the first larval ins tar in spite of the presence of SXL levels concordant with normal fema le development. The results of these experiments are discussed in ligh t of their relevance to the established observations that (1) the SXL made by the early promoter inhibits the hypertranscription of run at t he blastoderm stage, and (2) severe disturbances in SXL function (loss in XX individuals and gain in haplo-X individuals) result in lethalit y during embryogenesis while loss of msl function kills males much lat er.