Isolation and characterization of mutations that affect nuclear migration for dikaryosis in Coprinus cinereus.

We identified 14 Coprinus cinereus variants defective in nuclear migration for dikaryosis after restriction enzyme-mediated integration mutagenesis. All the variants were able to donate nuclei but failed to accept nuclei in compatible matings. Of the 14 variants, six were due to single-gene mutations. We characterized num1, a gene responsible for one of the mutations. The num1 ORF interrupted by three introns is predicted to encode a protein of 217 amino acids. The Num1 protein has two leucine-zipper motifs, one at the N-terminal region and the other at the C-terminal region. Num1 is also predicted to form a coiled-coil structure in its C-terminal half. The num1-1 mutant allele is predicted to produce a truncated peptide with only one leucine-zipper motif at the N-terminal region. A plasmid carrying a sequence for the truncated peptide could inhibit nuclear migration when introduced into a wild-type strain, showing that the num1-1 mutant allele is dominant negative. The transcription of num1 is down-regulated when the pathway regulated by B mating-type genes is activated.