Chromatin structure restricts origin utilization when quiescent cells re-enter the cell cycle.

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• Cellular quiescence (G0 phase) is a state of suspended cell growth and proliferation.
• Quiescent cells remain viable and can re-enter the cell cycle upon receiving external signals.
• Understanding the transition from quiescence back to proliferation is crucial for cell cycle regulation.

Purpose of the Study:

• To investigate the molecular mechanisms governing DNA replication initiation in quiescent cells re-entering the cell cycle.
• To identify factors and chromatin modifications that regulate the re-entry into S phase from G0.
• To determine if quiescent yeast cells are adequately licensed for DNA replication upon growth resumption.

Main Methods:

• Utilized a budding yeast model to study cellular quiescence and cell cycle re-entry.
• Quantified levels of replication initiation factors in quiescent and G1 phase cells.
• Analyzed S phase duration and the number of activated replication origins.
• Examined chromatin structure around replication origins using techniques assessing H3 occupancy and nucleosome positioning.
• Investigated the binding of the MCM licensing factor's Mcm4 subunit to origins.

Main Results:

• Quiescent cells exhibit significantly lower levels of replication initiation factors, delaying S phase entry.
• Re-entry into S phase involves a longer duration and activation of fewer replication origins compared to G1 cells.
• Inactive replication origins in quiescent cells show increased histone H3 occupancy and reduced nucleosome positioning.
• These chromatin changes inhibit the binding of the MCM complex (Mcm4 subunit) to origins of replication.

Conclusions:

• Quiescent yeast cells are under-licensed for DNA replication upon re-entry into the cell cycle.
• Chromatin modifications in G0 cells create a barrier to efficient replication origin licensing and activation.
• The study elucidates a novel regulatory mechanism controlling DNA replication during the transition from quiescence to proliferation.