Froeliger E H, Leong S A
Plant Disease Resistance Research Unit, U.S. Department of Agriculture-Agricultural Research Service, Madison, WI.
Gene. 1991 Apr;100:113-22. doi: 10.1016/0378-1119(91)90356-g.
Two unlinked incompatibility loci, a and b, control mating, pathogenicity, and sexual development in Ustilago maydis, a fungal pathogen of corn. Fusion of nonpathogenic haploid cells occurs when alleles of the a locus differ; fusion products that differ at the b locus will be pathogenic and complete sexual development. The two alleles of the a locus have been cloned. The a2 allele was isolated by exploiting the close linkage of the a locus to the genetic marker pan1. Several cosmids from a clonal a2 mating-type library allowed, via DNA-mediated transformation, an a1 mating type, pan1-1 auxotrophic recipient to grow prototrophically on medium lacking pantothenic acid. Cosmids containing functional a2 mating-type regions were identified by their ability to confer a2 mating behavior to the a1 recipient strain. The a1 allele was isolated from a plasmid library using DNA from the a2 region as a hybridization probe. The isolated clones contain a1 or a2 mating-type specificity as demonstrated by a plate mating assay, by pathogenicity tests on corn, and by gene replacement experiments. The a1 and a2 mating-type clones contain nonhomologous DNA segments that are flanked by similar nucleotide (nt) sequences. Restriction fragments containing a mating-type function are embedded within the nonhomologous DNA segments. U. maydis contains a single copy of either the a1 or a2 mating-type sequence within each haploid genome. This structural organization demonstrates that the a mating-type alleles of U. maydis are idiomorphs sensu Metzenberg and Glass (1990), nt sequences mapping to the same genetic locus that share no obvious evolutionary relationship.
两个不连锁的不相容基因座a和b，控制着玉米真菌病原体玉米黑粉菌（Ustilago maydis）的交配、致病性和有性发育。当a基因座的等位基因不同时，非致病性单倍体细胞会发生融合；在b基因座上不同的融合产物将具有致病性并完成有性发育。a基因座的两个等位基因已被克隆。a2等位基因是通过利用a基因座与遗传标记pan1的紧密连锁而分离出来的。来自克隆的a2交配型文库的几个黏粒，通过DNA介导的转化，使a1交配型、pan1-1营养缺陷型受体在缺乏泛酸的培养基上以原养型生长。含有功能性a2交配型区域的黏粒是通过它们赋予a1受体菌株a2交配行为的能力来鉴定的。a1等位基因是使用来自a2区域的DNA作为杂交探针，从质粒文库中分离出来的。通过平板交配试验、对玉米的致病性测试以及基因替换实验证明，分离出的克隆含有a1或a2交配型特异性。a1和a2交配型克隆包含非同源DNA片段，其两侧是相似的核苷酸（nt）序列。含有交配型功能的限制性片段嵌入在非同源DNA片段内。在每个单倍体基因组中，玉米黑粉菌只含有a1或a2交配型序列的单拷贝。这种结构组织表明，玉米黑粉菌的a交配型等位基因是Metzenberg和Glass（1990）意义上的特异型，即映射到同一遗传位点但没有明显进化关系的nt序列。
