Drosophila minichromosome maintenance 6 is required for chorion gene amplification and genomic replication

Duplication of the eukaryotic genome initiates from multiple origins of DNA replication whose activity is coordinated with the cell cycle. We have been studying the origins of DNA replication that control amplification of eggshell (chorion) genes during Drosophila oogenesis. Mutation of genes required for amplification results in a thin eggshell phenotype, allowing a genetic dissection of origin regulation. Herein, we show that one mutation corresponds to a subunit of the minichromosome maintenance (MCM) complex of proteins, MCM6. The binding of the MCM complex to origins in G1 as part of a prereplicative complex is critical for the cell cycle regulation of origin licensing. We find that MCM6 associates with other MCM subunits during amplification. These results suggest that chorion origins are bound by an amplification complex that contains MCM proteins and therefore resembles the prereplicative complex. Lethal alleles of MCM6 reveal it is essential for mitotic cycles and endocycles, and suggest that its function is mediated by ATP. We discuss the implications of these findings for the role of MCMs in the coordination of DNA replication during the cell cycle.