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Chromosome association of minichromosome maintenance proteins in Drosophila endoreplication cycles
1Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, California 94143-0448, USA.
Abstract:
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.
Insights
Minichromosome maintenance (MCM) proteins oscillate chromosome binding during DNA replication in Drosophila endocycles. This binding cycle occurs independently of mitosis, driven by cyclin E and erased by DNA synthesis.
Area of Science:
- Cell Biology
- Molecular Biology
- Genetics
Background:
- Minichromosome maintenance (MCM) proteins are crucial for eukaryotic DNA replication.
- MCM protein binding to chromatin oscillates with the cell cycle, linking replication to mitosis and preventing duplicate chromosome duplication.
- This coupling is absent in Drosophila endoreplication cycles (endocycles), which involve DNA replication without intervening mitoses.
Purpose of the Study:
- To investigate MCM protein behavior in endoreplicating Drosophila larval salivary glands.
- To determine if MCM-chromosome localization oscillates during endocycle S phases.
Main Methods:
- Examined MCM protein localization in polytene nuclei of Drosophila larval salivary glands.
- Investigated the role of cyclin E in MCM association with chromosomes.
- Assessed the effect of DNA synthesis on MCM-chromosome binding.
Main Results:
- MCM proteins were found in both chromosome-associated and dissociated states within polytene nuclei.
- Cyclin E was shown to induce chromosome association of DmMCM2.
- DNA synthesis was demonstrated to eliminate MCM-chromosome association.
Conclusions:
- Oscillations in MCM-chromosome binding can occur independently of mitosis.
- MCM-chromosome association cycles are proposed to occur during endocycles.
- The study presents a model where endocycle-specific cyclin expression uncouples MCM-chromosome association cycles from mitosis.