SummaryA bacterial artificial chromosome (BAC) library consisting of 11 000 clones with an average DNA insert size of 125 kb was constructed from rice nuclear DNA. The BAC clones were stable in E. coliafter 100 generations of serial growth, Transformation of the BAC clones by electroporation into E. coil was highly efficient and increased with decreasing size of the DNA inserts. The library was evaluated for the presence of organellar, repeated, and telomeric sequences. A very low percentage (<0.3%) of the library consisted of chloroplast and mitochondrial clones. Eighteen BACs were identified that hybridized with an Arabidopsis telomere repeat. Sixteen BAGs hybridized with the AA genomespecific repetitive sequence pOs48. Twelve clones were isolated that hybridized with three DNA markers linked to the Xa-21 disease resistance locus. The results indicate that the BAC system can be used to clone and manipulate large pieces of plant DNA efficiently.

Six molecular markers were mapped to a 7.4-cM region of rice chromosome 11 containing the Xa21 gene, which confers resistance to the pathogen Xanthomohas oryzae pv oryzae. Three markers, RG103, 248 and 818, co-segregated with Xa21 in a population of 1141 plants. Multiple copies of all marker loci were present within the region that was introgressed from Oryza longistaminata into O. sativa. The marker loci were cloned and primers were designed that defined sequence-tagged sites. Physical mapping of the three tightly linked central markers revealed that RG103, the marker that hybridizes to the Xa21 gene, resides on a separate DNA fragment from the other two markers.