Preferential homogenization between adjacent and alternate subrepeats in wheat rDNA.

DNA from the "non-transcribed spacer" (NTS) of two wheat ribosomal RNA gene (rDNA) clones was sequenced. The regions flanking the internal subrepeat arrays are highly conserved between the two clones; the nucleotide sequence differ by less than one-half percent. In contrast, the consensus sequences of the subrepeats in the two arrays differ by three percent. Mutations unique to each array, yet found in more than one subrepeat of the array, are preferentially found in adjacent and alternate subrepeats. The similarity of the DNA sequences of the flanking regions is consistent with a model of homogenization among rDNA gene units by intergenic conversion. We propose that a different mechanism, preferential conversion between neighboring subrepeats, is largely responsible for the homogenization of subrepeats within an array.