A transcript identified by MuA of maize is associated with Mutator activity

A Mu element, which we designated MuA, was cloned from a maize line with a Mutator background by its homology to the terminal inverted repeats of Mu1. Like other Mu elements, MuA has terminal inverted repeats of approximately 200 bp which are homologous to those of Mu1, but the internal region is different. MuA is unique in several aspects, being approximately 5.5 kb in size and the largest Mu element reported to date. It is flanked by 8 bp duplications instead of the 9 bp duplications found in most other Mu insertions. The internal sequences of MuA hybridize to restriction fragments that cosegregate with Mutator activity in maize lines showing 1:1 segregation for somatic mutability. The most interesting observation is that a transcript of approximately 3.5 kb identified by the internal sequences of MuA is both qualitatively and quantitatively associated with Mutator activity. This transcript is present in maize lines containing germinal Mutator activity but is undetectable in maize inbreds with no known Mutator activity. The amount of the transcript is decreased in lines that have lost germinal Mutator activity. Northern analysis of maize a1-Mum mutant lines that segregate 1:1 for mutability shows that a transcript of the same size is associated with somatic Mutator activity. These data suggest that the 3.5 kb transcript is produced by the autonomous element that confers both germinal and somatic Mutator activity. The possibility that MuA is an autonomous or regulator element of the Mu family is discussed.