DNA damage induces persistent cell cycle delays in budding
yeast.
Allan LeSage, Steve Kron
Center for Molecular
Oncology, The University of Chicago, 924 E. 57th St., Chicago, IL 60637,
USA
Microscopic observations of cell division delay in budding
yeast following DNA damage have been reported since the 1950's. Genetic
analysis of DNA damage response identified the RAD9 pathway which
both detects DNA damage and enforces mitotic delay. Checkpoint mutants
confer catastrophic division after DNA damage. To make detailed
observations of wildtype and mutant DNA damage responses, we developed
an automated time-lapse microscopy system with integral delivery of
psoralen/UV-mediated DNA damage that allows quantitative analysis of
cell division kinetics. We analyzed cell division patterns in DNA-damaged wildtype, rad9 and rad9 rad17 rad24
strains. Time-lapse analysis both confirms and extends the classical
model, exposing new functions of checkpoint genes and repair in the DNA-damage recovery. Compared to the dose-dependence of cell division delay
in wildtype, the rad9 rad17 rad24 strain displayed
a small but significant delay after damage. Surprisingly, marked
checkpoint delays in wildtype and mutants persist multiple cell
divisions after the DNA damaging event. 0.5 sec exposure to 365 nm UV in
the presence of 10 µg/ml 8-methoxy psoralen during S phase increased
wildtype cell cycle length from 60 to 90 min but the next cell cycle
required 150 min. Suggesting that repair influences both first and
second cycle delays, pso2 mutants deficient in excision of
psoralen demonstrated no first division delay at 0.5 sec but the second
division increased to 220 min.
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