RNA codons and protein synthesis. 15. Dissimilar responses of mammalian and bacterial transfer RNA fractions to messenger RNA codons.

Abstract Unfractionated aminoacyl-tRNA preparations from guinea pig liver and Escherichia coli respond similarly to most, but not to all, mRNA codons. To clarify the dissimilar responses, liver and E. coli aminoacyl-tRNA preparations were fractionated by column chromatography, and the effects of trinucleoside diphosphate codons upon the binding of purified aminoacyl-tRNA fractions to ribosomes were determined. Sixteen species of mammalian aminoacyl-tRNA were found corresponding to seven amino acids. Codon-anticodon relationships for six amino acids were compared. Only seven universal species of aminoacyl-tRNA were found with both E. coli and liver aminoacyl-tRNA. Seven species of aminoacyl-tRNA were obtained from liver that were not detected with E. coli preparations; conversely, five species of aminoacyl-tRNA were obtained from E. coli that were not found with liver preparations. The results also suggest that some organisms may contain little or no aminoacyl-tRNA for certain codons. For example, liver isoleucine-tRNA responds well to AUA; no response was detected with E. coli isoleucine-tRNA. Liver arginine-tRNA responds well to AGG, but was deficient in response to AGA; E. coli arginine-tRNA was relatively deficient in response to AGA and AGG. One species of E. coli cysteine-tRNA recognizes UGU, UGC and UGA; under identical conditions liver cysteine-tRNA responds to UGU and UGC, but not to UGA. A species of serine-tRNA responding to UAG was obtained from E. coli CR63, but not from E. coli B. These findings are discussed in terms of tRNA suppressors and possible mechanisms for regulating protein synthesis. Mammalian and bacterial aminoacyl-tRNA fractions respond to sets of codons according to wobble base-pairing. Another pattern of degeneracy, in which A, G or U may occupy the third position of synonym triplets, was observed with two peaks of E. coli serine-tRNA.