Molecular analysis of cloned human 18S ribosomal DNA segments

A fraction of DNA from the human fetal lung fibroblast line IMR-90, 30-fold enriched for ribosomal DNA, was cloned in the lambda phage vector Charon 16A. Of 978 clones assayed by hybridization to a mixture of 125I-labeled 18S and 28S ribosomal RNA, 11 recombinants containing a 3.8-megadalton segment of human 18S ribosomal DNA were identified. Restriction endonuclease analysis of these clones demonstrated variation only in orientation of the human gene segment within the phage vector. Restriction sites that we had previously detected from analysis of restriction products of unfractionated human DNA by using the Southern transfer method were also present in the cloned DNA segment. Recombinant DNA technology thus provides a valid and efficient means to define structural conservation or variation within families of human genes.