Quadruplet reading by a +1-frameshifting tRNA. (A) The hisD nucleotide sequence in Salmonella contains the +1-frameshifting window ACC-CCU-GAA (coding for Thr-Pro-Glu). The hisD3018 mutant of Salmonella harbors a single C insertion to the CCU codon, resulting in the sequence of the +1-frameshifting window ACC-CCC-UGA (coding for Thr-Pro-stop). This hisD3018 mutant is suppressed by the +1-frameshifting tRNA SufB2, which reads the quadruplet codon CCCU, thus restoring the original hisD reading frame of protein synthesis (for Thr-Pro-Glu). (B) Sequence and cloverleaf structure of Salmonella ProM, ProL, and SufB2, each in the native-state with the full complement of all natural post-transcriptional modifications. Note that SufB2 is derived from ProL by the insertion of an extra G37a nucleotide in the anticodon loop, resulting in an expanded ASL.