Sequence requirements for efficient translational frameshifting in the Escherichia coli dnaX gene and the role of an unstable interaction between tRNA(Lys) and an AAG lysine codon.

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Abstract
Synthesis of the gamma-subunit of DNA polymerase III holoenzyme depends on precise and efficient translational frameshifting to the -1 frame at a specific site in the dnaX gene of Escherichia coli. In vitro mutagenesis of this frameshift site demonstrated the importance of an A AAA AAG heptanucleotide sequence, which allows two adjacent tRNAs to retain a stable interaction with mRNA after they slip to the -1 position. The AAG lysine codon present in the 3' half of this heptanucleotide was a key element for highly efficient frameshifting. A tRNA(Lys) with a CUU anticodon, which has a strong affinity for AAG lysine codons, is present in eukaryotic cells but absent in E. coli. Expression in E. coli of a mutant tRNA(Lys) with a CUU anticodon specifically inhibited the frameshifting at the AAG codon, suggesting that the absence of this tRNA in E. coli contributes to the efficiency of the dnaX frameshift.