NUC-2, A COMPONENT OF THE PHOSPHATE-REGULATED SIGNAL-TRANSDUCTION PATHWAY IN NEUROSPORA-CRASSA, IS AN ANKYRIN REPEAT PROTEIN

In response to phosphorus limitation, the fungus Neurospora crassa syn thesizes a number of enzymes that function to bring more phosphate int o the cell. The NUC-2 protein appears to sense the availability of pho sphate and transmits the signal downstream to the regulatory pathway. The nuc-2(+) gene has been cloned by its ability to restore growth of a nuc-2 mutant under restrictive conditions of high pH and low phospha te concentration. We mapped the cloned gene to the right arm of linkag e group II, consistent with the chromosomal position of the nuc-2 muta tion as determined by classical genetic mapping. The nuc-2(+) open rea ding frame is interrupted by five introns and codes for a protein of 1 066 amino acid residues. Its predicted amino acid sequence has high si milarity to that of its homolog in Saccharomyces cerevisiae, PHO81. Bo th proteins contain six ankyrin repeats, which have been implicated in the cyclin-dependent kinase inhibitory activity of PHO81. The phenoty pes of a nuc-2 mutant generated by repeat-induced point mutation and o f a strain harboring a UV-induced nuc-2 allele are indistinguishable. Both are unable to grow under the restrictive conditions, a phenotype which is to some degree temperature dependent. The nuc-2(+) gene is tr anscriptionally regulated. A 15-fold increase in the level of the nuc- 2(+) transcript occurs in response to a decrease in exogenous phosphat e concentration.