Decoding of tandem quadruplets by adjacent tRNAs with eight-base anticodonloops

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 s tudy 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 UA GA quadruplet, and release factor I is partially inactivated, the tandem UA GAs specify two leucines with an overall efficiency of at least 10%. The pr esence of a purine at anticodon loop position 32 of the tRNA decoding the c odon 5' to the first UAGA seems to influence translation of the following c odon. Another finding is intraribosomal dissociation of anticodons from cod ons and their re-pairing to mRNA at overlapping or nearby codons, In one ca se where GCC is replaced by CGG, only a single Watson-Crick base pair can f orm upon re-pairing when decoding is resumed. This has implications for the mechanism of some cases of programmed frameshifting.