Genomic organization of the canrep repetitive DNA in Brassica juncea

Canrep is a heterogeneous, tandemly repeated, 176 bp nucleotide sequence that contains a single Hind III site and is present in high copy numbers in the genomes of many Brassica species. Complete clusters of repeats of this DNA were cloned from the nuclear DNA of Brassica juncea. Restriction-fragment dimers and higher multimers of the 176 bp sequence have arisen by mutations within the Hind III recognition sequence. Adjacent repeats from within the same cluster usually have different nucleotide sequences with features indicating that diversity is generated by a mechanism that causes site-specific base substitutions. While most of the units of canrep DNA are clustered in long arrays of tandem repeats, some are dispersed throughout the genome as isolated copies or in small clusters. Regardless of the size of the arrays, each cluster begins and ends with a variable-length, truncated repeat and is flanked by inverted copies of the sequence 5-ATCTCAT3-,which is not part of the basic sequence of the canrep family of DNAs. Furthermore, some clusters are located close to nucleotide sequences related to those of known plant transposons. Thus, canrep elements may be dispersed by transposition. There are two distinct subfamilies of canrep sequences in B. juncea, and one of these is closely related to one of the two subfamilies of this type of DNA from B. napus, indicating that it originated from B. campestris, the common diploid ancestor of both amphidiploid species. Neither the repetitive DNA nor nucleotide sequences flanking canrep clusters are transcribed in seedlings, suggesting that even small arrays of repeats are located in heterochromatic regions and might be involved in chromatin condensation and/or chromosome segregation.