A gene-enriched BAC library for cloning large allele-specific fragments from maize: isolation of a 240-kb contig of the bronze region.

A generic bacterial artificial chromosome (BAC) library from a complex plant genome like maize may not be suitable for some types of genomic analysis, for example, for establishing correlations between the genetic and the physical organization of a given chromosome region. Previously, we carried out extensive genetic analysis of the bronze (Bz) region in Zea mays using a W22 inbred line carrying the Bz-McC allele; however, BAC libraries of that line are neither available nor under construction. Here, we report the isolation of large, adjacent BAC clones of this region from a partial BAC library of W22. We developed a BAC vector suitable for cloning NotI fragments and used it to clone size-fractionated genomic DNA that had been cut to completion with the methylation-sensitive, rare-cutting enzyme NotI. This strategy resulted in a very significant enrichment of large genic DNA. From a library of about 20,000 BACs, containing just two-thirds of a maize genome, we isolated 16 BAC clones of the 110-kb distal Bz fragment and 10 BAC clones of the 130-kb proximal Bz fragment. This recovery means that our strategy resulted in a 15- to 24-fold enrichment of specific sequences. The order of the BAC clones in the 240-kb contig, predetermined from an internal NotI site in the Bz-McC allele was confirmed by hybridization with sequences from sites previously mapped proximal and distal to Bz and by sequencing. To show the general utility of our approach and the value of our partial BAC library, we also isolated BAC clones of other sequences, such as tub4 and the complex R-r allele, contained in the same size fraction of DNA. This is the first report of the use of a BAC vector to clone allele-specific large DNA fragments from a plant with a large genome, circumventing the need to construct a complete BAC library.