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J. Cell Biol.,
Volume 140, Number 3, February 9, 1998 451-460
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.
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