nimO, an Aspergillus gene related to budding yeast Dbf4, is required for DNA synthesis and mitotic checkpoint control

The nimO predicted protein of Aspergillus nidulans is related structurally and functionally to Dbf4p, the regulatory subunit of Cdc7p kinase in buddin g yeast, nimOp and Dbf4p are most similar in their C-termini, which contain a PEST motif and a novel, short-looped Cys(2)-His(2) zinc finger-like moti f, DNA labelling and reciprocal shift assays using ts-lethal nimO18 mutants showed that nimO is required for initiation of DNA synthesis and for effic ient progression through S phase. nimO18 mutants abrogated a cell cycle che ckpoint linking S and M phases by segregating their unreplicated chromatin, This checkpoint defect did not interfere with other checkpoints monitoring spindle assembly and DNA damage (dimer lesions), but did prevent activatio n of a DNA replication checkpoint, The division of unreplicated chromatin w as accelerated in cells lacking a component of the anaphase-promoting compl ex (bimE(APC1)), consistent with the involvement of nimO and APC/C in separ ate checkpoint pathways. A nimO deletion conferred DNA synthesis and checkp oint defects similar to nimO18. Inducible nimO alleles lacking as many as 2 44 C-terminal amino acids supported hyphal growth, but not asexual developm ent, when overexpressed in a ts-lethal nimO18 strain. However, the truncate d alleles could not rescue a nimO deletion, indicating that the C terminus is essential and suggesting some type of interaction among nimO polypeptide s.