STREPTOMYCIN, SUPPRESSION, AND THE CODE.

We have found that an external agent, streptomycin, can upset the genetic code, producing specific misreadings during in vitro polypeptide synthesis. This interference is at the level of the ribosone-messenger RNA-sRNA complex, for a modification in the ribosome makes the in vitro system insensitive to this effect. Streptomycin' is a bacteriocidal agent. :It is a basic molecule that can bind strongly to nucleic acids.2 It interferes with and finally blocks protein synthesis while permitting continued RNA and DNA synthesis.3 The mechanism of its killing is not known, but the existence of single mutations to high-level resistance suggests a unitary cause, a single, vital point of attack. Spotts and Stanier4 hypothesized that the ribosomes were the sensitive elements, and work with the in vitro system, in which the poly U-directed incorporation of phenylalanine was shown to be inhibited by streptomycin,51 6 further implicated the ribosomes as the site of the shift from sensitivity to resistance. The sensitivity to streptomycin has just been shown by Davies7 and by Cox, White, and Flaks8 to reside on the 30s subunit of the 70s ribosome. If these 30s subunits are taken from a sensitive strain, the reconstructed in vitro system is sensitive, while if they are derived from a resistant strain, the system is resistant. The Effects of Streptomycin on the Specificity of Amino Acid Incorporation.-Although the incorporation of phenylalanine into hot TCA-insoluble material is blocked (often by 50-75%) by streptomycin, the incorporation of other amino acids, not normally coded for by poly U, is stimulated, Table 1 shows that, using a purified system, the incorporation of isoleucine (UUA, UAA, CAU),9 and to a much lesser extent serine (UUC, UCC, AGC) and leucine (UUA, UUC, UUG, UCC), is stimulated in the presence of streptomycin, The same is true for crude extracts (Table 2). Tyrosine (UUA) is not noticeably stimulated, nor are the other amino acids. Asparagine (CAA, CUA, UAA) was examined by damping the incorpo