Genetic control of phosphorus assimilation in Neurospora crassa : dose-dependent dominance and recessiveness in constitutive mutants

Mutants called nuc-1c, constitutive for alkaline phosphatase synthesis, were isolated and mapped very close to nuc-1 mutants in which this enzyme is not expressed. nuc-1 is epistatic to nuc-1c. nuc-1c acts only if it is cis to normal nuc-1 function. The preparation of partial diploids heterozygous for various nuc-1 alleles is described; nuc-1c is dominant to nuc-1+, which in turn is dominant to nuc-1. In heterocaryons with nuc-1+, nuc-1c is dominant when it is present in high proportion, but essentially recessive if it is present in low proportions. In heterocaryons with nuc-1, nuc-1c is again dominant when present in high proportions, but in low proportions it "complements" to give essentially normal repressibility. A model of regulation consistent with these findings is presented.