Putative homologs of SSK22 MAPKK kinase and PBS2 MAPK kinase of Saccharomyces cerevisiae encoded by os-4 and os-5 genes for osmotic sensitivity and fungicide resistance in Neurospora crassa

We cloned and characterized Neurospora NcSSK22 and NcPBS2 genes, similar to yeast SSK22 mitogen-activated protein (MAP) kinase kinase kinase and the PBS2 MAP kinase kinase genes, respectively. Disruptants of the NcSSK22 gene were sensitive to osmotic stress and resistant to iprodione and fludioxonil. Their phenotypes were similar to those of osmotic-sensitive (os) mutants os-1, os-2, os-4, and os-5. The os-4 mutant strain transformed with the wild-type NcSSK22 gene grew on a medium containing 4% NaCl and was sensitive to iprodione and fludioxonil. In contrast, the NcPBS2 gene complemented the osmotic sensitivity and fungicide resistance of the os-5 mutant strain. We sequenced the NcPBS2 gene of the os-5 mutant strain (NM216o) and found five nucleotides deleted within the kinase domain. This result suggests that the gene products of os-4 and os-5 are components of the MAP kinase cascade, which is probably regulated upstream by two-component histidine kinase encoded by the os-1/nik1 gene.