The genes for 18S, 5.8S and 28S ribosomal RNA of Bombyx mori are organized into tandem repeats of uniform length.

The organization of the multiple genes for 18S, 5.8S and 28S rRNA in the genome of the silkworm, Bombyx mori was determined by restriction endonuclease digestion and Southern blot hybridization. The ribosomal genes (rDNA) are tandemly reiterated, with a uniform repeat length of 6.9 . 10(6) daltons. Each rDNA repeat has a single site for EcoRI, HindIII, HpaI and SmaI and each of these sites has been mapped with respect to the others and to the rRNA genes; each repeat consists of a transcribed region (6 . 10(6)daltons) containing the 18S, 5.8S and 28S rRNA genes (5' leads to 3') and also a small non-transcribed spacer (approximately 10(6) daltons). Complete rDNA repeats were cloned using the vector RSF2124 and grown in Escherichia coli. Characterization of the rDNA plasmids confirmed the conclusions from studies of the total rDNA. The organization of B. mori rDNA is similar to that of other eukaryotes, except for the absence of heterogeneity in the rDNA repeat length; thus, there is neither variation in the length of the non-transcribed spacer nor the presence of inserts in a detectable portion of the rDNA. The utility of this map, and particularly of the rDNA plasmids, for detailed studies of rRNA transcription and processing is discussed.