X-ray-induced specific-locus mutations in the ad-3 region of two-component heterokaryons of XXXX . XII. Analysis of multiple-locus ad-3 mutations reveals a nonrandom distribution of the separate sites of recessive lethal damage throughout the genome.

@entity1 @entity19 @entity5536 @entity1980 @entity74 @entity238
Previous studies on X-ray-induced @entity5536 -3 mutations induced in heterokaryon 12 of @entity1980 showed that they consisted of gene/point mutations, multilocus deletion mutations, and 3 different genotypic classes of multiple-locus mutations (designated [-3]IR + RLCL, ad-3R + RLCL, and ad-3R + RL). In the present paper, multiple-locus mutations consisting of gene/point mutations at the ad-3A or the ad-3B locus with sites of recessive lethal damage closely linked to the ad-3 region (designated ad-3R + RLCL) or with sites of recessive lethal damage elsewhere in the genome (designated ad-3R + RL) were analyzed to determine whether they resulted from mutations at the same sites or different sites throughout the genome. It was assumed that if the recessive lethal mutations in individual multiple-locus mutations showed complementation on @entity5536 -supplemented medium, they resulted from mutations at different sites. Multiple-locus mutations from both major genotypic classes were combined, as forced heterokaryons, in all possible pairwise combinations and then were plated out on @entity5536 -supplemented medium. These studies indicated that 89.3% (50/56) of the recessive lethal mutations in these 2 classes of multiple-locus mutations complement one another. Thus, they are presumed to have resulted predominantly from @entity74 at different sites throughout the genome. Within the group of 20 multiple-locus mutations that did not complement in various pairwise combinations, 90% (18/20) appear to map in a region, distal to the ad-3 region, defined by a series of overlapping multilocus deletion mutations in 6 mutations of genotype ad-3R + RLCL. The other 10% (2/20) are located elsewhere on Linkage Group I or elsewhere in the genome. The present data base on multiple-locus mutations is unique; such events either can not be detected, or can only be detected with difficulty, in other eukaryotic specific-locus assay systems such as @entity1 cells in culture, @entity238 or @entity19 . Our data on X-ray-induced ad-3 specific-locus mutations from the present and previous studies demonstrate the presence of additional sites of @entity74 , both closely linked with the ad-3 region or elsewhere in the genome, in ad-3 specific-locus mutations. Because the frequencies of each class of multiple-locus mutations is dose-dependent, they must be taken into account in genetic risk assessment exercises. Failure to acknowledge the presence of such additional sites of @entity74 in the utilization of specific-locus data could result in underestimation of the risk of @entity1 exposure to environmental mutagens.
