Removing the Block to Pre-meiotic DNA Re-replication in S.
cerevisiae.
Michael Mallory (1), Katrina Cooper (2), Michal
Jarnik (1), Randy Strich (1)
(1) Institute for Cancer
Research, Fox Chase Cancer Center, 7701 Burholme Ave, Philadelphia, PA
19111, USA; (2) Department of Biochemistry, Hahnemann University,
Philadelphia PA
The precise transmission of the genome at each
generation is essential for the survival of an organism. In budding
yeast, overlapping regulatory systems mediated by cyclin B-cyclin
dependent kinase (Cdk) activity restrict mitotic S phase to only once
per cell cycle. This study demonstrates that ectopic overexpression of
the B-type cyclin Clb1p can recruit approximately 30% of meiotic cells
to re-initiate DNA replication and undergo 2-4 additional divisions
producing asci containing up to 20 spores. The efficiency of re-replication was directly related to Clb1p levels indicating that this
cyclin-Cdk activity is rate limiting. Further characterizations revealed
that the additional rounds of DNA replication occur prior to nuclear
division and were independent of the Cdk inhibitor Sic1p, an important
inhibitor of pre-meiotic S phase. The resulting spore clones are viable,
haploid, and demonstrated Mendellian marker segregation indicating that
DNA replication was complete and chromosomes segregated normally. This
latter observation is consistent with our finding that these cell
divisions were coordinated and contained a spindle checkpoint. Since
ectopic expression of Clb1p does not alter cell cycle progression in
mitotic cells, these results indicate that the checkpoint pathways that
block re-replication are different in meiotic and mitotic cells.
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