The Identification of the Ribosomal Rna Cistron

Despite the fact that ribosomal ribonucleic acid constitutes the bulk (85%) of cellular RNA, its mode of origin is little understood. While clearly not exhaustive , two alternatives can, at present, be entertained. One, would assume a DNA-dependent r e a ~ t i o n l-~ and the other would invoke a synthetic mechanism independent of DNA. The fact that the base composition of ribosomal RNA shows no tendency to correlate4-with homologous DNA is irrelevant to a choice between the two hypotheses. The presumed DNA segment involved might be so small as to constitute a statistically inadequate sample of the overall base composition. Posing the problem in the form of these two alternatives suggests the following question, pertinent to a decision and amenable to experimental resolution : Does DNA contain a sequence complementary to homologous ribosomal RNA? to questions of complementarity is in principle provided by Hall and Spjegelman's6 demonstration that specific hybrid formation can be exhibited between T2-DNA and the RNA synthesized in E. coli infected with T2. These experiments used labeled nucleic acids for ease of identification and took advantage of equilibrium centrifugation in density gradients' to separate free RNA from that which hybrid-ized to DNA. The technical difficulties inherent in using hybridization to establish the existence of complementarity between ribosomal RNA and some sequence in the DNA have already been discussed.6 The major complications stem from the numerology of the situation. For example, the 235 RNA component of the ribosomes is 1.1