A single p34^cdc2 protein kinase (encoded by nimX^cdc2) is required at G₁ and G₂ in Aspergillus nidulans

We have cloned and sequenced a homolog of cdc2 from Aspergillus nidulans that can complement the Schizosaccharomyces pombe cdc2-33 mutation. The gene was deleted and is required for continued nuclear DNA replication but not for mitochondrial DNA replication. Three different temperature-sensitive alleles were generated by reverse genetics. All of the mutations generate the nim phenotype of A. nidulans. The new gene was designated nimX^cdc2 as it is not allelic to any of the other nim genes (nimA to nimW) of A. nidulans. Reciprocal shift experiments place an essential function for nimX^cdc2 in G₁ and G₂. Antipeptide antibodies were generated that detect NIMX^cdc2, and antisera were also generated to detect NIME^cyclinB. The two p34^cdc2 protein species previously detected in A. nidulans, p34 and p37, both precipitate using NIMX^cdc2 C-terminus-specific antibodies but only p34 co-precipitates with NIME^cyclinB. Dephosphorylation of denatured p34 converts it to the p37 form, showing p37 to be the non-phosphorylated form of NIMX^cdc2. The phosphorylation of p34 is therefore associated with its interaction with NIME^cyclinB.