Genetic Fine Structure of the BRONZE Locus in Maize

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The bronze (bz) locus in maize, located in the short arm of chromosome 9 (9S), is the structural gene for the anthocyanin biosynthetic enzyme UFGT. The gene has been cloned and its physical map has been oriented relative to the centromere of 9S. We report here the genetic fine structure mapping of several biochemically characterized EMS-induced bz-E mutations, derived from the Bz-W22 isoallele, and Ds insertion bz-m mutations, derived from the Bz-McC isoallele. Two UFGT-, CRM+ mutants (bz-E2 and bz-E5), which genetically identify coding sequences in the gene, and three UFGT-, CRM- bz-E mutants were mapped against the Ds insertion mutants bz-m1 and bz-m2(DI) by selecting Bz intragenic recombinants from heterozygotes of the type bz-E/bz-m . The exclusive occurrence of one recombinant outside marker class allowed the unambiguous placement of the mutants in a genetic fine structure map. Peculiarly, the two CRM+ bz-E mutants lie upstream of the three CRM- bz-E mutants and at a considerable genetic distance. The UFGT allozymes encoded by the progenitor alleles Bz-W22 and Bz-McC differ in two properties, thermal stability and activity. The sites responsible for these properties were mapped as unselected markers among the Bz intragenic recombinants. The thermal stability site, which also identifies a coding region of the gene, mapped very close to the CRM+ bz-E mutant sites. The site responsible for variation in activity, which probably identifies a region involved in regulation of expression of the bz locus, mapped at the 5' or proximal end of the locus. It was found to be inseparable from the Ds insertion in bz-m1 that lies very close to the 5' end of the transcribed region.—Evidence was obtained that the insertion of Ds within the bz gene has a suppressing effect on intragenic recombination. Additional data are also presented supporting our observation that Ds affects the pattern of intragenic recombination at bz.—Based on the total genetic length of the bz gene and on the physical size of the transcribed region, we estimate that one unit of recombination at bronze corresponds to 14 kb of DNA. This estimate is more than 100 times smaller than the average value for the whole genome and implies that there may be regions, such as bronze, that serve as hotspots for recombination.

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