A new repetitive DNA region was identified in the non-transcribed spacer of human rDNA, namely a long (4.6kb) sequence motif (XbaI element) was present in two copies. The repeating unit composed of two parts. One of them consisted of unique nucleotide sequences, interrupted by some simple sequences. The other, about 3.1kb long one assembled only from highly repeated simple sequences. The unique sequence region contained two, inverted copies of the human AluI type repetitive DNA family. The authors suggest that the XbaI elements may flank the tandem arrays of human rRNA genes as terminal repeats and they might function both as the origin of rDNA replication and/or site of homologous recombination.
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