Genetic Isolation, Cloning, and Analysis of a Mutator-Induced, Dominant Antimorph of the Maize amylose extender1 Locus.

We report the genetic identification, molecular cloning, and characterization of a dominant mutant at the amylose extender1 locus, Ae1-5180. The identities of our clones are corroborated by their ability to reveal DNA polymorphisms between seven wild-type revertants from Ae1-5180 relative to the Ae1-5180 mutant allele and between four of five independently derived, Mutator (Mu)-induced recessive ae1 alleles relative to their respective wild-type progenitor alleles. The Ae1-5180 mutation is associated with two Mu1 insertions flanked by complex rearrangements of ae1-related sequences. One of the Mu1 elements is flanked by inverted repeats of ae1-related DNA of at least 5.0 kb in length. This Mu1 element and at least some of this flanking inverted repeat DNA are absent or hypermethylated in six of seven wild-type revertants of Ae1-5180 that were analyzed. The second Mu1 element is flanked on one side by the 5.0-kb ae1-specific repeat and on the other side by a sequence that does not hybridize to the ae1-related repeat sequence. This second Mu1 element is present in revertants to the wild type and does not, therefore, appear to affect ae1 gene function. A 2.7-kb ae1 transcript can be detected in wild-type and homozygous ae1-Ref endosperms 20 days after pollination. This transcript is absent in endosperms containing one, two, or three doses of Ae1-5180. This result is consistent with a suppression model to explain the dominant gene action of Ae1-5180 and establishes Ae1-5180 as an antimorphic allele. Homozygous wild-type seedlings produce no detectable transcript, indicating some degree of tissue specificity for ae1 expression. Sequence analyses establish that ae1 encodes starch branching enzyme II.