We have analyzed multiple recombinant DNA clones containing ribosomal RNA repeat units of the silkmoth, Bombyx mori. In combination with genomic DNA blots, analysis of these clones indicated that the rDNA repeat of B. mori is 10.8 kilobase pair in length and tandemly repeated in the genome, as reported by Manning et al. (18). However, contrary to that report, approximately 12% of the rDNA cistrons are interrupted by insertions of non-ribosomal DNA. Two classes of DNA insertions were identified. In one class the insertions are positioned in a region of the 28S coding sequence similar to that of the predominant rDNA insertions found in a variety of Dipteran and Tetrahymena species. In the second class, probable insertions are found close to the 3' terminus of the 28S coding sequence. Restriction enzyme analysis indicates that the two classes of insertions are not related.
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