Cholorate toxicity in Aspergillus nidulans: the selection and characterisation of chlorate resistant mutants.

Mutation in at least ten genes can result in chlorate reistance in Aspergillus nidulans. Mutation in seven of these genes also results in the inability to use nitrate as nitrogen source. The various classes of resistant mutant obtained occur in different proportions, depending on whether or not a mutagenic treatment is employed, and also on which nitrogen source is used for selection. The prinicipal effect of mutagen arises because mutations in the niaD gene, the nitrate reductase structural gene, are relatively much commoner when no mutagen is used than after treatment with N-methyl-N'-nitro-N-nitrosoguanidine. This may be connected with the finding that deletions involving the niaD gene are relatively more common among samples of spontaneous niaD mutants. Some of these deletions extend to the neighbouring niiA gene, the structural gene for nitrite reductase. The selection procedures used were designed to avoid bias in favour of any particular chlorate resistant phenotype. Even if biases existed however, these could not account for the variation found from nitrogren source to nitrogen source in the proportions of certain resistant classes having apparently identical chlorate resistance phenotypes.