A tandemly repeated DNA sequence is associated with both knob-like heterochromatin and a highly decondensed structure in the meiotic pachytene chromosomes of rice

Highly repetitive tandem DNA sequence repeats are often associated with cen tromeric and telomeric regions of eukaryotic chromosomes. The rice tandem r epeat Os48 is organized as long arrays of a 355 bp monomer and is mainly lo cated in the telomeric regions. The chromosomal locations of the Os48 seque nce were determined by fluorescence in situ hybridization (FISH) on rice pa chytene chromosomes. The majority of the Os48 loci are associated with brig htly 4',6-diamidino-2-phenylindole (DAPI)-stained and knob-like hererochrom atin in rice pachytene chromosomes. As with other DNA sequences located in the heterochromatic regions, the cytosines of the CG and C(A/T)G sites with in the Os48 repeat are heavily methylated. Surprisingly, a proportion of th e FISH signals are highly decondensed and deviate significantly from the DA PI-stained periphery of the pachytene chromosomes. This highly decondensed chromatin structure has not been reported in pachytene chromosomes prepared from alcohol/acid-fixed meiotic samples in any other eukaryotic species. T he condensation of the Os48 sequences is dynamic during prophase I of meios is. The FISH signals derived from the Os48 repeat progress from a condensed configuration between leptonema and early pachynema into a decondensed str ucture from middle pachynema to diakinesis, and then return to a condensed form at metaphase I.