Intragenic location of an unstable insertion element within gene b5 of the fungus Ascobolus immersus

Summary A fine structure map of gene b5 has been established in Ascobolus immersus and the unstable mutant site b5-301 (phenotype: ascospore coloration) has been found to map within this gene. This map was constructed using seven b5 mutants induced by ICR170 and is based on the additivity of recombinant frequencies and confirmed by three point tests. The unstable site 301 is located between the induced sites. In particular, mutant 249 is located to the left of site 301, whereas sites 601 and 754 are located to the right.Previous studies showed that the inducing gene of mutant b5-301 reversions are either closely linked to the b5 locus or within it in certain strains. The study of asci resulting from reciprocal recombination between unstable mutant site and several induced mutant sites showed that neither crossovers located on the left nor the right of site 301, separate the unstable site from the inducing gene. Thus, the inducing gene was found to map within gene b5 as did the inducible site.These results constitute a genetic argument showing the presence of an insertion element. In this case, the insertion structure contains at least the integration site (inducible site) and the inducing gene which allows the excision.