Codon-anticodon recognition and transfer RNA utilization for the leucine tRNA isoaccepting species of Escherichia coli have been studied by protein synthesis in vitro directed by sequenced bacteriophage MS2 RNA. We have added radioactive Leu-tRNALeu isoaccepting species as tracers, rather than use a tRNA-dependent system, since in the presence of an excess of non-radioactive leucine, there is no transfer of radioactive leucine from one isoaccepting species to another. MS2-specific peptides containing leucine residues encoded by known codons were isolated and identified, and the relative abilities of the Leu-tRNALeu isoaccepting species to transfer leucine into these peptides compared. Sequenced tRNA1Leu and sequenced tRNA3Leu are of roughly equal efficiency in their ability to recognize CUC and CUA codons, while tRNA3Leu is highly preferred for the CUU codon; tRNA4Leu and tRNA5Leu both recognize UUA and UUG codons, with tRNA4Leu slightly preferred for the UUA codon. We conclude that: (1) wobble is greater than permitted by the wobble hypothesis; (2) there is still some discrimination in the third code letter, and that the CUX4 (CUC, CUA, CUU, CUG) portion of the leucine family of six codons is not read by a simple "two out of three" mechanism; (3) a Watson-Crick pair (C · G) between codon and anticodon does not appear to be preferred over an unorthodox pair (C · C) in the wobble position; (4) a standard wobble pair (U · G) between codon and anticodon is preferred over an unorthodox pair (U · C); and (5) the extensive wobble observed in the CUX4 leucine codon series is not paralleled in the UUX4 leucine (UUG, UUA) and phenylalanine (UUU, UUC) codon series, where mistranslation would be the consequence of such wobble.
All Science Journal Classification (ASJC) codes
- Structural Biology
- Molecular Biology