Ribosomal Intergenic Spacers Are Filled with Transposon Remnants

Eukaryotic ribosomal DNA (rDNA) comprises tandem units of highly conserved coding genes separated by rapidly evolving spacer DNA. The spacers of all 12 species examined were filled with short direct repeats (DRs) and multiple long tandem repeats (TRs), completing the rDNA maps that previously contained unannotated and inadequately studied sequences. The external transcribed spacers also were filled with DRs and some contained TRs. We infer that the spacers arose from transposon insertion, followed by their imprecise excision, leaving short DRs characteristic of transposon visitation. The spacers provided a favored location for transposon insertion because they occupy loci containing hundreds to thousands of gene repeats. The spacers' primary cellular function may be to link one ribosomal RNA transcription unit to the next, whereas transposons flourish here because they have colonized the most frequently used part of the genome.