| Abstract: | The restriction endonuclease EcoR1 cleaves Saccharomyces cerevisiae DNA, which codes for ribosomal RNA (rRNA), into seven fragments, A second restriction endonuclease, HindIII, cleaves the same yeast ribosomal DNA into two fragments. These two restriction enzymes each yield DNA segments that total about 5.9 megadaltons. The "repeat unit" of the yeast genes coding for rRNA is thus about 5.9 megadaltons or about 9000 base pairs long. The two HindIII-cleaved DNA fragments as well as one of the EcoR1-cleaved DNA fragments were purified and amplified by cloning in Escherichia coli. Three of the seven EcoR1-generated DNA fragments could then be ordered by treating the two cloned HindIII DNA fragments with EcoR1. This led the assignment of the two HindIII restriction sites. The various restriction DNA fragments were hybridized directly from the gel utilizing 32P-labeled 5 S, 5.8 S, 18 S, and 25 S rRNA. Identification of the various DNA restriction segments then led to the final ordering of the DNA fragments. The gene coding for the 5 S RNA is adjacent to the gene coding for the 35 S precursor rRNA. These two groups of genes thus occur as a cluster in the following sequence: 5 S-spacer]-spacer-18 S-5.8 S-25 S-spacer]-spacer-5 S]. The actual map of the DNA restriction fragments is presented. |
