Visualization of the terminal structure of rice chromosomes 6 and 12 with multicolor FISH to chromosomes and extended DNA fibers

High-resolution fluorescence in situ hybridization (FISH) on interphase and pachytene nuclei, and extended DNA fibers enabled microscopic distinction of DNA sequences less than a few thousands of base pairs apart. We applied this technique to reveal the molecular organization of telomere ends in jap onica rice (Oryza sativa ssp. japonica), which consist of the Arabidopsis t ype TTTAGGG heptameric repeats and the rice specific subtelomeric tandem re peat sequence A (TrsA). Southern hybridizations of DNA digested with Bal31 and EcoRI, and FISH on chromosomes and extended DNA fibers demonstrated tha t (1) all chromosome ends possess the telomere tandem repeat measuring 3-4 kb; (2) the subtelomeric TrsA occurs only at the ends of the long arms of c hromosomes 6 and 12, and measure 6 and 10 kb, which corresponds to 231 and 682 copies for these sites, respectively; (3) the telomere and TrsA repeats are separated by at most a few thousands of intervening nucleotide sequenc es. The molecular organization for a general telomere organization in plant chromosomes is discussed.