DOSAGE COMPENSATION IN DROSOPHILA - THE X-CHROMOSOMAL BINDING OF MSL-1 AND MLE IS DEPENDENT ON SXL ACTIVITY

In Drosophila, dosage compensation, i.e. the equalization of levels of X-linked gene products in the two sexes, is achieved by the hypertranscription of most X-linked genes in males relative to females. The products of at least four genes, collectively termed male-specific lethal (msl) genes, are required for this process and, at least in the case of three of them, mediate this function through an association with the X chromosome in males. We have studied some of the parameters that affect the association of the msl-1 gene product and found that its presence is dependent on the wildtype function of the other three genes, leading to the conclusion that these gene products contribute to the formation of a multi-subunit complex. Furthermore, the X-chromosomal association of the msl-1 and mle gene products is negatively correlated with the level of function of the master regulatory gene Sxl and can assume either a mosaic or a uniform distribution in the tissues of mutant XX individuals. Surprisingly, we also found that the association of these two msl gene products with the two X chromosomes in females of certain mutant genotypes does not result in the hypertranscription of X-linked genes or in any apparent reduction in viability.