The highly recombinogenic bz locus lies in an unusually gene-rich region of the maize genome

The bronze (bz) locus exhibits the highest rate of recombination of any gen e in higher plants. To investigate the possible basis of this high rate of recombination, we have analyzed the physical organization of the region aro und the bz locus. Two adjacent bacterial artificial chromosome clones, comp rising a 240-kb contig centered around the Bz-McC allele, were isolated, an d 60 kb of contiguous DNA spanning the two bacterial artificial chromosome clones was se quenced. We find that the bz locus lies in an unusually gene- rich region of the maize genome. Ten genes, at least eight of which are sho wn to be transcribed, are contained in a 32-kb stretch of DNA that is unint errupted by retrotransposons. We have isolated nearly full length cDNAs cor responding to the five proximal genes in the cluster. The average intertran script distance between them is just 1 kb, revealing a surprisingly compact packaging of adjacent genes in this part of the genome. At least II small insertions, including several previously described miniature inverted repea t transposable elements, were detected in the introns and 3 ' untranslated regions of genes and between genes. The gene-rich region is flanked at the proximal and distal ends by retrotransposon blocks. Thus, the maize genome appears to have scattered regions of high gene density similar to those fou nd in other plants. The unusually high rate of intragenic recombination see n in bz may be related to the very high gene density of the region.