Checkpoint defects leading to premature mitosis also cause endoreplicationof DNA in Aspergillus nidulans

The G2 DNA damage and slowing of S-phase checkpoints over mitosis function through tyrosine phosphorylation of NIMXcdc2 in Aspergillus nidulans. We de monstrate that breaking these checkpoints leads to a defective premature mi tosis followed by dramatic rereplication of genomic DNA. Two additional che ckpoint functions, uvsB and uvsD, also cause the rereplication phenotype af ter their mutation allows premature mitosis in the presence of low concentr ations of hydroxyurea. uvsB is shown to encode a rad3/ATR homologue, wherea s uvsD displays homology to rad26, which has only previously been identifie d in Schizosaccharomyces pombe. uvsB(rad3) and uvsD(rad26) have G2 checkpoi nt functions over mitosis and another function essential for surviving DNA damage. The rereplication phenotype is accompanied by lack of NIMEcyclinB, but ectopic expression of active nondegradable NIMEcyclinB does not arrest DNA rereplication. DNA rereplication can also be induced in cells that ente r mitosis prematurely because of lack of tyrosine phosphorylation of NIMXcd c2 and impaired anaphase-promoting complex function. The data demonstrate t hat lack of checkpoint control over mitosis can secondarily cause defects i n the checkpoint system that prevents DNA rereplication in the absence of m itosis. This defines a new mechanism by which endoreplication of DNA can be triggered and maintained in eukaryotic cells.