Structure and variation of human ribosomai DNA: molecular analysis of cloned fragments (Recombinant DNA; moderately repetitive DNA: DNA polymorphism; rRNA processing; R-loops)

SUMMARY Eco-RI-A fragments of the human ribosomal RNA gene family from two types of tissue and three individuals were cloned in h vectors and compared by restriction enzyme digestion and electron microscopy. The EcoRl fragment A contains (i) 0.2 kb of the 3’ end of the 18s rDNA, (ii) 2.5 kb of internal transcribed spacer and the 5.8s rDNA, and (iii) 4.6 kb of the 28s rDNA gene. All of the six cloned rDNA fragments isolated are identical by these analyses. Moreover, all contain a H&c11 site that is absent in about 50% of the rDNA identified by genomic blotting. Polymorphism in the nontranscribed spacer rDNA was studied on genomic blots of BamHEdigested DNA, using the 3’ end of the 28s rDNA as a probe. The boundaries between the 18s rDNA, internal transcribed spacer, 28s rDNA, and external nontranscribed spacer were determined by R-loop analysis, further defining the organization of the ribosomal RNA precursor.