LARGE TANDEM DUPLICATION ASSOCIATED WITH A MU2 INSERTION IN ZEA-MAYS B-PERU GENE

The b locus of Zea mays encodes a transcriptional activator of the ant hocyanin biosynthetic pathway. The B-Peru allele is expressed in the a leurone layer of the seed, which results in dark purple pigmentation o f this tissue. An unstable Mutator-induced B-Peru mutant allele, b-Per um220, displays weak, variable pigment and a high germinal reversion r ate not characteristic of other Mutator insertions. Characterization o f relevant regions of b-Perum220 revealed a Mu2 element insertion in o ne copy of a 534 bp sequence. This 534 bp sequence is tandemly triplic ated in the progenitor B-Peru allele, upstream of the B-Peru transcrip tion start site. In addition to the Mu2 insertion, the b-Perum220 alle le contains a newly formed large tandem duplication of 4.0 kb, which i ncludes the promoter region and the first three exons of the B-Peru ge ne. The Mu2 element does not reside at any of the duplication breakpoi nts. The molecular study of eleven independent germinal revertants rev ealed five structural classes including structures in which the 4.0 kb tandem duplication is partially or completely deleted, the Mu2 elemen t is partially or completely deleted, or a combination of these events has occurred. We hypothesize that most of the revertants arose by une qual recombination between the duplicated regions. Based on these stru ctural analyses, models are discussed to explain the reduced b gene ex pression in b-Perum220.