Checkpoint Defects Leading to Premature Mitosis Also Cause Endoreplication of DNA inAspergillus nidulans

Abstract

The G2 DNA damage and slowing of S-phase checkpoints over mitosis function through tyrosine phosphorylation of NIMX^cdc2inAspergillus nidulans. We demonstrate that breaking these checkpoints leads to a defective premature mitosis followed by dramatic rereplication of genomic DNA. Two additional checkpoint functions,uvsB and uvsD, also cause the rereplication phenotype after their mutation allows premature mitosis in the presence of low concentrations of hydroxyurea.uvsB is shown to encode a rad3/ATRhomologue, whereas uvsD displays homology torad26, which has only previously been identified inSchizosaccharomyces pombe. uvsB^rad3anduvsD^rad26have G2 checkpoint functions over mitosis and another function essential for surviving DNA damage. The rereplication phenotype is accompanied by lack of NIME^cyclinB, but ectopic expression of active nondegradable NIME^cyclinBdoes not arrest DNA rereplication. DNA rereplication can also be induced in cells that enter mitosis prematurely because of lack of tyrosine phosphorylation of NIMX^cdc2and impaired anaphase-promoting complex function. The data demonstrate that lack of checkpoint control over mitosis can secondarily cause defects in the checkpoint system that prevents DNA rereplication in the absence of mitosis. This defines a new mechanism by which endoreplication of DNA can be triggered and maintained in eukaryotic cells.