Dm-myb mutant lethality in Drosophila is dependent upon mip130: positive and negative regulation of DNA replication.

Gene amplification at the chorion loci in Drosophila ovarian follicle cells is a model for the developmental regulation of DNA replication. Previously, we showed that the Drosophila homolog of the Myb oncoprotein family (DmMyb) is tightly associated with four additional proteins and that DmMyb is required for this replication-mediated amplification. Here we used targeted mutagenesis to generate a mutant in the largest subunit of the DmMyb complex, the Aly and Lin-9 family member, Myb-interacting protein 130 (Mip130). We found that mip130 mutant females are sterile and display inappropriate bromodeoxyuridine (BrdU) incorporation throughout the follicle cell nuclei at stages undergoing gene amplification. Whereas mutations in Dm-myb are lethal, mutations in mip130 are viable. Surprisingly, Dm-myb mip130 double mutants are also viable and display the same phenotypes as mip130 mutants alone. This suggests that Mip130 activity without DmMyb counteraction may be responsible for the Dm-myb mutant lethality. RNA interference (RNAi) to selectively remove each DmMyb complex member revealed that DmMyb protein levels are dependent upon the presence of several of the complex members. Together, these data support a model in which DmMyb activates a repressive complex containing Mip130 into a complex competent to support replication at specific loci in a temporally and developmentally proscribed manner.