Quadruplet codons: implications for code expansion and the specification of translation step size

Decoding of tandem quadruplets by adjacent tRNAs with eight-base anticodon loops

To expand the genetic code for specification of multiple non-natural amino acids, unique codons for these novel amino acids are needed. As part of a study of the potential of quadruplets as codons, the decoding of tandem UAGA quadruplets by an engineered tRNA(Leu) with an eight-base anticodon loop, has been investigated. When GCC is the codon immediately 5' of the first UAGA quadruplet, and release factor 1 is partially inactivated, the tandem UAGAs specify two leucines with an overall efficiency of at least 10%. The presence of a purine at anticodon loop position 32 of the tRNA decoding the codon 5' to the first UAGA seems to influence translation of the following codon. Another finding is intraribosomal dissociation of anticodons from codons and their re-pairing to mRNA at overlapping or nearby codons. In one case where GCC is replaced by CGG, only a single Watson-Crick base pair can form upon re-pairing when decoding is resumed. This has implications for the mechanism of some cases of programmed frameshifting.