DNA CLASS ORGANIZATION ON MAIZE ADH1 YEAST ARTIFICIAL CHROMOSOMES

The organization of higher plant genomes is poorly understood. These g enomes are typified by their large size and extensive repetitive DNA c omponent. To further our understanding of the composition and arrangem ent of genomic DNA sequences, we have performed a detailed analysis of a contiguous interval of 280 kb surrounding the Adh1 locus of maize. A series of overlapping A subclones was isolated, and individual fragm ents were characterized with respect to their genomic copy number. Cro ss-hybridization analyses were used to define a minimum of 37 repetiti ve DNA classes within the 280-kb interval. Hybridizations with highly repetitive DNAs cloned from other regions of the maize genome suggeste d that >50% of all highly repetitive elements in maize are represented on this single yeast artificial chromosome. These repeated sequences were found in an organizational pattern not previously observed; indiv idual repetitive elements are interspersed with one another in an appa rently random fashion and are spatially separate from single copy numb er sequences. Extensive tandem arrays were not found. Sequences from o ne end of the 280-kb interval were used to isolate overlapping yeast a rtificial chromosome clones, representing the first step in a chromoso me walk.