Isolation and some properties of a mammalian ribosomal DNA.

The DNA coding for 28 S and 18 S ribosomal RNA, including the spacer regions, has been isolated from calf (Bos taurus) thymus gland. The method used included shearing of the total DNA to a highly homogeneous size population, selective heat denaturation and S1 nuclease treatment to remove single stranded DNA. Repeated centrifugation on density gradients yields a 140-fold purified rDNA fraction with a GC content of 61.2% Eco RI nuclease cleaves this DNA into two fragments of 16.4 and 4.9 X 10(6) daltons. Hybridization of these fragments with 28 S and 18 S rRNA shows that the 28 S coding sequence is located mostly on the 4.9 X 10(6) dalton fragment, while both the 16.4 and 4.9 X 10(6) dalton fragments contain the 18 S sequence. The data indicate that the ribosomal RNA gene has a repeat unit of 21.3 X 10(6) daltons which includes a nontranscribed spacer of about 12.5 X 10(6) daltons.