Mu3 may cause chromosomal rearrangements at a distance from the insertion site

--Yvonne Thorstenson and Michael Freeling

Adh1-3F1124 is a mutation at the Adh1-3F locus caused by a Mu3 insertion at the TATA box within the promoter region upstream of the gene. This mutation confers a tissue-specific alteration in ADH1 expression; the mutant plant exhibits 6% of normal activity in the scutellum, but 100% activity in pollen (Chen et al., Genetics 116:469-477, 1987). Several derivatives of the mutant, selected via allyl alcohol resistant pollen grains, are null in both pollen and scutellum (Chen, Ph.D. dissertation, UC Berkeley, 1986).

In an effort to determine the structural changes at the locus that resulted in a null phenotype, Southern analyses of some of the null derivatives recovered by Chen were performed. At least two derivatives had restriction map differences that were not immediately adjacent to the Mu3 insertion even though Mu3 was still present at its original location. One derivative had a 1.3kb insertion about 1kb downstream from Mu3. A second derivative had changes in a region beginning at about 3kb downstream from Mu3 . In neither case were the changes associated with the new insertion of a transposon carrying Mu ends.

This phenomenon, if widespread, could cause trouble for persons attempting to tag genes with Mu. If Mu routinely causes such rearrangements in distant regions, it may knock out genes far from its insertion site. The Mu element responsible for the mutant gene would still cosegregate with the mutational lesion, but the Mu-containing clone could completely miss the mutant gene.

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