On the mechanism of ribosomal frameshifting at hungry codons

In a few, rather rare cases, frameshift mutant alleles are phenotypically suppressed during limitation for particular aminoacyl-tRNA species. The simplest interpretation is compensatory ribosome frameshifting at a "hungry" codon in the vicinity of the suppressed frameshift mutation. We have now tested this interpretation directly by obtaining amino acid sequence data on such a phenotypically suppressed protein. We used a plasmid-borne lacZ gene, engineered to be in the (+) reading frame. Its background leakiness is increased by two orders of magnitude during lysyl-tRNA limitation. The enzyme made under this condition has the amino acid sequence expected from the DNA sequence up to the first lysine codon, then shifts in the (-) direction to recreate the correct lacZ reading frame. The lysine is replaced by serine, presumably due to cognate reading of an overlapping AGC codon displaced by one base to the 3' side of the AAG codon. When the 3' overlapping codon is AGA or AGG, there is no ribosome frameshifting; when it is AGU (read by the same serine tRNA) there is frameshifting, although less efficiently than in the case of AGC. The mechanism of cognate overlapping reading contradicts more elaborate models that two of the authors have suggested previously. However, the possibility remains that there is more than one mechanism of ribosome frameshifting at hungry codons.