Drosophila follicle cell amplicons as models for metazoan DNA replication: A cyclinE mutant exhibits increased replication fork elongation

Abstract

This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 25, 2006. Gene clusters amplified in the ovarian follicle cells of Drosophila serve as powerful models for metazoan DNA replication. In response to developmental signals, specific genomic regions undergo amplification by repeated firing of replication origins and bidirectional movement of replication forks for Ϸ50 kb in each direction. Previous work focused on initiation of amplification, defining replication origins, establishing the role of the prereplication complex and origin recognition complex (ORC), and uncovering regulatory functions for the Myb, E2F1, and Rb transcription factors. Here, we exploit follicle cell amplification to investigate the control of DNA replication fork progression and termination, poorly understood processes in metazoans. We identified a mutant in which, during gene amplification, the replication forks move twice as far from the origin compared with wild type. This phenotype is the result of an amino acid substitution mutation in the cyclinE gene, cyclinE 1f36 . The rate of oogenesis is normal in cyclinE 1f36 /cyclinE Pz8 mutant ovaries, indicating that increased replication fork progression is due to increased replication fork speed, possibly from increased processivity. The increased amplification domains observed in the mutant imply that there are not replication fork barriers preventing replication forks from progressing beyond the normal 100-kb amplified region. These results reveal a previously unrecognized role for CyclinE in controlling replication fork movement.gene amplification ͉ oogenesis ͉ fork progression

Cyclin E controlsDrosophilafemale germline stem cell maintenance independently of its role in proliferation by modulating responsiveness to niche signals

“…2B; data not shown), indicating that CycE is also required for division of GSC daughters. By contrast, hypomorphic CycE 1F36 (Park et al, 2007) GSCs retain sufficient activity to support normal proliferation rates. CycE functions in a complex with a cyclin-dependent kinase, Cdk2, to regulate the cell cycle (Möröy and Geisen, 2004); therefore, we asked whether Cdk2 was also required for GSC proliferation.…”

‘Poking’ microtubules bring about nuclear wriggling to position nuclei

“…S1). The commencement of nuclear drifting coincides with the time when mitoses cease and the follicle cells start to become polytenic (Dej and Spradling, 1999;Park et al, 2007). Remarkably, nuclear migration and wriggling come to an end at the same time.…”

“…4). Apparently, there is no sign of a welldefined MTOC in the follicle cells from stage 6 on when they start to become polytenic (Park et al, 2007) and start to drift along the apical-basal axis. However, as anti-c-tubulin labeling revealed, there is a profound cortical enrichment of the MTOCs that may well seed MTs throughout the egg cortex (Fig.…”

‘Poking’ microtubules bring about nuclear wriggling to position nuclei

“…Apparently, there is no sign of a well defined MTOC in the follicle cells from stage 6 on when they start to become polytenic (Park et al, 2007) and start to drift along the apical-basal axis.…”

“…The difference between the two positions is significant (P<0.001) and clearly shows that the follicle cell nuclei drift along the apical-basal axis during stages 6 to 9 of egg primordia development. polytenic (Dej and Spradling, 1999;Park et al, 2007). Remarkably, nuclear migration and wriggling come to an end at the same time.…”