Analysis of a Drosophila amplicon in follicle cells highlights the diversity of metazoan replication origins

To investigate the properties of metazoan replication origins, recent studies in cell culture have adopted the strategy of identifying origins using genome-wide approaches and assessing correlations with such features as transcription and histone modifications. Drosophila amplicon in follicle cells (DAFCs), genomic regions that undergo repeated rounds of DNA replication to increase DNA copy number, serve as powerful in vivo model replicons. Because there are six DAFCs, compared with thousands of origins activated in the typical S phase, close molecular characterization of all DAFCs is possible. To determine the extent to which the six DAFCs are different or similar, we investigated the developmental and replication properties of the newly identified DAFC-34B. DAFC-34B contains two genes expressed in follicle cells, although the timing and spatial patterns of expression suggest that amplification is not a strategy to promote high expression at this locus. Like the previously characterized DAFC-62D, DAFC-34B displays origin activation at two separate stages of development. However, unlike DAFC-62D, amplification at the later stage is not transcription-dependent. We mapped the DAFC-34B amplification origin to 1 kb by nascent strand analysis and delineated cis requirements for origin activation, finding that a 6-kb region, but not the 1-kb origin alone, is sufficient for amplification. We analyzed the developmental localization of the origin recognition complex (ORC) and the minichromosome maintenance (MCM) 2-7 complex, the replicative helicase. Intriguingly, the final round of origin activation at DAFC-34B occurs in the absence of detectable ORC, although MCMs are present, suggesting a new amplification initiation mechanism.