THE STR120 SATELLITE DNA OF SOYBEAN - ORGANIZATION, EVOLUTION AND CHROMOSOMAL SPECIFICITY

A highly repeated DNA sequence family, STR120, with tandemly arranged repetitive units (monomers) of approximately 120 bp, has been identifi ed in soybean [Glycine maw(L.) Merr.]. Five related clones showing tan dem repeats of a 120-bp-long monomer were isolated from a soybean geno mic library. Results of Southern blotting experiments using three of t he clones as probes onto genomic DNA digested with different restricti on enzymes were in agreement with a tandem arrangement of these sequen ces in the genome. A total of 12 monomers were sequenced, showing cons iderable sequence heterogeneity. A consensus sequence of 126 bp was ob tained that exhibits an average similarity of 81% to the sequenced uni ts. In three of the clones identified, neighbouring units are signific antly more similar to each other than to units from different clones; in the remaining two clones, however, similarity between the two units observed is low (70%), while the overall similarity between the two c lones is high (95%). This indicates that in these cases the repetitive unit may be the dimer rather than the monomer. Based on the presence of direct repeats within each monomer, we suggest that the 120-bp mono mer may itself have evolved by duplication of an ancestral 60-bp unit. The STR120 family distribution is limited to annual soybeans and is n ot found, at least at high-copy number, in related perennial soybeans or other members of the tribe Phaseolae. Fluorescence in situ hybridiz ation (FISH) to metaphase chromosomes using four of the clones as prob es shows that the number of chromosomal locations differs depending on the stringency conditions and goes from two to eight when the stringe ncy is progressively lowered. The estimated copy number for one of the clones is from 5000 to 10 000, but this may just represent a lower bo undary for the whole family in consideration of the high sequence dive rgence observed within the family. FISH and sequence analysis therefor e indicate that different subfamilies as well as higher-order repeat u nits are present in the STR120 family, very much like those in primate alpha satellite DNA, and that some of the subfamilies seem to exhibit divergence on a chromosomal basis.