Isolation and characterization of rat ribosomal DNA clones

Four EcoRI fragments, which contain the transcribed portion of the rat rDNA repeat, have been isolated from a rat genome library cloned in lambda Charon 4A vector. Three of the fragments, 9.6, 6.7, and 4.5 kb, from clones lambda ChR-B4, lambda Nr-42, and lambda ChR-C4B9, contained part of the 5'-NTS, the 5'-ETS, 18S rDNA, ITS-1, 5.8S rDNA, 28S rDNA and approximately 3.5 kb of the 3'-NTS. Two EcoRI fragments, from clones lambda ChR-B4 and lambda ChR-B7E12, which coded for the 5'-NTS, the ETS, and most of the 18S rDNA, differed by 1 kb near the EcoRI site upstream of the 5' terminus of 18S rRNA. Restriction maps of the cloned DNA fragments were constructed by cleavage of the fragments with various restriction endonucleases and Southern hybridization with 18S, 5.8S, and 28S rRNA. These maps were confirmed and extended by subcloning several regions of the repeat in pBR322.