EXTENSIVE, NONRANDOM DIVERSITY OF EXCISION FOOTPRINTS GENERATED BY DS-LIKE TRANSPOSON ASCOT-1 SUGGESTS NEW PARALLELS WITH V(D)J RECOMBINATION

Upon insertion, transposable elements can disrupt or alter gene functi on in various ways. Transposons moving through a cut-and-paste mechani sm are in addition often mutagenic when excising because repair of the empty site seldom restores the original sequence. The characterizatio n of numerous excision events in many eukaryotes indicates that transp oson excision from a given site can generate a high degree of DNA sequ ence and phenotypic variation. Whether such variation is generated ran domly remains largely to be determined. To this end, we have exploited a well-characterized system of genetic instability in the fungus Asco bolus immersus to perform an extensive study of excision events. We sh ow that this system, which produces many phenotypically and geneticall y distinct derivatives, results from the excision of a novel Ds-like t ransposon, Ascot-1, from the spore color gene b2. A unique set of 48 m olecularly distinct excision products were readily identified from a r epresentative sample of excision derivatives. Products varied in their frequency of occurrence over 4 orders of magnitude, yet most showed s mall palindromic nucleotide additions. Based on these and other observ ations, compelling evidence was obtained for intermediate hairpin form ation during the excision reaction and for strong biases in the subseq uent processing steps at the empty site. Factors likely to be involved in these biases suggest new parallels between the excision reaction p erformed by transposons of the hAT family and V(D)J recombination. An evaluation of the contribution of small palindromic nucleotide additio ns produced by transposon excision to the spectrum of spontaneous muta tions is also presented.