Forward and back mutation in thepyr3region of Neurospora. I. Mutations from arginine dependence to prototrophy

Abstract

A system for the screening of reversions in an arginine-dependent strain of Neurospora has been developed.Reversion resulted from any one of three events:(a) Back mutation of the original arg gene.(b) Forward mutation at the pyr locus. This has the pleiotropic effect of suppressing arg while determining a pyrimidine requirement. Interaction with the unmutated nuclei of the same conidium permits growth on minimal medium.(c) Forward mutation at the pyr locus to alleles of intermediate activity. These alleles suppress arg, but do not create a pyrimidine requirement.

Induction of Forward Mutants in the pyr-3 Region of Neurospora

“…In the present system such one-chain hits should yield mosaic clones (mutational mosaics) containing arg pyr and arg pyr+ nuclei derived respectively from the mutated and the unmutated strand. The expectation that these heterokaryons would be detectable by the usual screening method is based on the following observations : ( a ) the suppressor effect of pyr is dominant over pyr+ (Reissig, 1958); ( b ) mutations decreasing pyr+ activity to one half are readily scored by the present method (Reissig, 1963); (c) arg pyrlarg pyr+ mosaics originated by mutation of one nucleus in multinucleated macroconidia were in fact recovered (Reissig, 1960) ; ( d ) when plating microconidia at high concentration, a sizable proportion of the mutant colonies analysed were arg pyrlarg pyr+ mosaics, probably as a result of contamination with the background microconidia.…”

“…radiation seems to produce more of the latter than either ethylmethane sulphonate or nitrous acid (Table 2). A closer examination of this difference demands classification of the mutants into the two expected genotypes : arg+pyr+ and arg prysu-arg, by recombination tests (Reissig, 1960). Briefly, the unknown strain is crossed to a co arg pgr tester, and the ensuing ascospores are plated on arginine medium.…”

“…irradiation ( Table 2) can also be understood. t Including 10 arg+ pyr+ and 2 arg p r r g from a previous study (Reissig, 1960).…”

“…A particularly promising forward selection method is available for the pyr-3 (pyr) locus of Neurospora. This method was described in the first paper of this series (Reissig, 1960), and will be henceforth called the suppressor method. It can be outlined as follows : uninucleate conidia (microconidia) from an arginine-dependent strain of Neurospora are plated on medium supplemented with pyrimidine, but lacking arginine.…”

“…Low plating densities and premature addition of the top layer resulted in low recoveries of mutants (Reissig, 1960;and unpublished (Reissig, 1960). Figure 1 a shows that U.V.…”