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© The Rockefeller University Press, 0021-9525/1998//451 $5.00
The Journal of Cell Biology, Volume 140, Number 3, , 1998 451-460

Article

Chromosome Association of Minichromosome Maintenance Proteins in Drosophila Endoreplication Cycles



Tin Tin Su and Patrick H. O'Farrell

Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, California 94143-0448

Minichromosome maintenance (MCM) proteins are essential eukaryotic DNA replication factors. The binding of MCMs to chromatin oscillates in conjunction with progress through the mitotic cell cycle. This oscillation is thought to play an important role in coupling DNA replication to mitosis and limiting chromosome duplication to once per cell cycle. The coupling of DNA replication to mitosis is absent in Drosophila endoreplication cycles (endocycles), during which discrete rounds of chromosome duplication occur without intervening mitoses. We examined the behavior of MCM proteins in endoreplicating larval salivary glands, to determine whether oscillation of MCM–chromosome localization occurs in conjunction with passage through an endocycle S phase. We found that MCMs in polytene nuclei exist in two states: associated with or dissociated from chromosomes. We demonstrate that cyclin E can drive chromosome association of DmMCM2 and that DNA synthesis erases this association. We conclude that mitosis is not required for oscillations in chromosome binding of MCMs and propose that cycles of MCM–chromosome association normally occur in endocycles. These results are discussed in a model in which the cycle of MCM–chromosome associations is uncoupled from mitosis because of the distinctive program of cyclin expression in endocycles.

Abbreviations used in this paper: BrdU, bromo-deoxyuridine; MCM, minichromosome maintenance; NEB, nuclear envelope breakdown; PBT, PBS + 0.2% Tween; rt, room temperature.

This project was funded by National Institute of Health (NIH) grant (R01-GM57193) to P.H. O'Farrell and an NIH postdoctoral fellowship (GM15032) to T.T. Su.

Address all correspondence to Patrick O'Farrell, Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, California 94143-0448. Tel.: (415) 476-4707/0525. Fax: (415) 502-5143/5145. E-mail: ofarrell{at}cgl.ucsf.edu; tintin{at}cgl.ucsf.edu


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