An unstable allele of the bronze 2 (bz2) locus was isolated from a Robertson's Mutator Zea mays line containing a family of active transposable Mu elements. This mutation is somatically unstable, resulting in numerous revertant purple sectors on a bronze kernel. By following the variegated kernel phenotype through two generations, several lineages have been identified that have a distorted transmission of the mutant phenotype (fewer variegated kernels are produced than expected). Southern blot analysis ofMu elements in these plants demonstrates a correlation between an inhibition of digestion of Mu elements by certain restriction enzymes and the loss of somatic reversion at the mutant allele. The DNA modification can occur in all the Mu elements in a plant within one generation; however, plants have been identified that contain both modified and unmodified elements, suggesting that the modification can occur in a progressive manner. We hypothesize that the DNA modification results in nonfunctional elements.The Mutator system of maize (Zea mays) We describe here the initial characterization of a Mutatorinduced mutation of bronze 2 (bz2), a gene encoding one of the last steps in the anthocyanin pathway (10). Plants have been identified that have a decreased transmission of the mutant variegated phenotype, and a molecular characterization of the Mu elements within these lines suggests that the elements are inactivated by extensive DNA modification. as indicated in figure legends. Unincorporated nucleotides were removed from nick-translated probes (18) by spin dialysis (19) through Bio-Gel P-10 (Bio-Rad) equilibrated in 10 mM Tris HCl, pH 7.5/1 mM EDTA. Maize DNA fragments for nick-translation were prepared by digestion with restriction enzymes, separated from the plasmid vector DNA by electrophoresis through low-melting agarose (FMC Seaplaque), extracted with phenol, and precipitated with ethanol. Hybridizations were in 50% (vol/vol) formamide (EM Science)/450 mM sodium chloride/45 mM sodium citrate/0.05% Ficoll 400 (Pharmacia)/0.05% polyvinylpyrrolidone/0.05% bovine serum albumin, containing salmon sperm DNA (Sigma) at 100 Ag/ml at 420C for 36-48 hr. Filters were washed in 15 mM sodium chloride/1.5 mM sodium citrate/0.1% NaDodSO4 at 650C (Mul and Adhl probes) or 500C (ars probe) for 3 hr with three buffer changes. The filters Abbreviations: Mu, members of a transposable element family in Mutator lines; ars, autonomously replicating sequence; kb, kilobase(s); bp, base pair(s); APT-paper, aminophenylthioether paper.
MATERIALS AND METHODS