NTRS, A NEW FAMILY OF HIGHLY REPETITIVE DNAS SPECIFIC FOR THE T1 CHROMOSOME OF TOBACCO

Species-specific repeated DNAs are important for identifying genomic c omponents of hybrid organisms in plant breeding and in taxonomic studi es, and we have previously described the HRS60 and GRS families of hig hly repetitive DNA sequences in tobacco. Here we describe a new family of highly repetitive DNA sequences termed NTRS (SspI family) that we have isolated from Nicotiana tomentosiformis (Goodspeed) and character ized and that is specific for the genomes of several species of the su bgenus Tabacum. In situ hybridization showed that NTRS sequences are p resent in three pairs of chromosomes of N. tomentosiformis, six pairs of chromosomes of N. kawakamii, and only one pair of chromosomes of N. tabacum at an intercalary site. The NTRS family is not present in the N. otophora genome. The majority of NTRS sequences appeared to be org anized in tandem arrays in which local DNA structures sensitive to sin gle strand-specific chemical probes, potassium per manganate, and osmi um tetroxide complexed with pyridine revealed a periodicity of 220 bp, equal to the length of the repeat unit. The inner cytosine in CCGG an d CC(A/T)GG sequences of the NTRS family is frequently methylated. Clo ned and sequenced NTRS monomeric units are 212-219 bp in length and sh ow 83.5%-95% mutual homology. They exhibit properties characteristic f or molecules that possess stable intrinsic curvature, but there are di fferences among individual monomers in the degree of curvature. NTRS s equences like HRS60 and GRS sequences, were found to specify nucleosom e positions.