MUTATOR INSERTIONS IN AN INTRON OF THE MAIZE KNOTTED1 GENE RESULT IN DOMINANT SUPPRESSIBLE MUTATIONS

The knotted1 (kn1) locus of maize is defined by a series of dominant m utations affecting leaf development. We recovered 10 additional mutant alleles in lines containing active Mutator transposable elements. Nin e of these alleles contain Mu1 or Mu8 elements inserted within a 310-b p region of the kn1 third intron. All five Mu8 insertions are in the s ame orientation whereas both orientations of Mu1 were recovered. North ern analysis showed that ectopic expression of kn1 within developing l eaves is correlated with the mutant phenotype for the four alleles ana lyzed. Transcript size was not altered. The effect of Mu activity, as measured by the extent of Mu element methylation or by the presence of the autonomous MuDR element, was investigated for two alleles. Kn1-mu m2, containing a Mu8 element, and Kn1-mum7, containing a Mu1 element, required Mu activity for the knotted phenotype. We examined the effect of Mu activity on ectopic kn1 expression in Kn1-mum2 and found that t he transcript was present in leaves of Mu active individuals only. We discuss possible mechanisms by which !Mu activity could condition kn1 gene expression.