Programmed ribosomal frameshifting generates the Escherichia coli DNA polymerase III γ subunit from within the τ subunit reading frame

摘要

The Escherichia coli dnaX gene encodes both the τ and γ subunits of DNA polymerase III holoenzyme in one reading frame. The 71.1 kDa τ and the shorter γ share N-terminal sequences. Mutagenesis of a potential ribosomal frameshift signal located at codons 428–430 without changing the amino acid sequence of the τ product, eliminated detectable synthesis of the γ subunit, suggesting that the reading frame is shifted at that sequence and γ is terminated by a nonsense codon located in the ?1 frame 3 nucleotides downstream of the signal. This seems to be the first known case of a frameshift which is used, along with the termination codon in the ?1 frame, to terminate a peptide within a reading frame. [Mutagenesis of a dibasic peptide (lys-lys) at codons 498–499, the site at which a τLacz fusion protein was cleaved in vitro (1) had no effect on γ formation in vivo, suggesting that cleavage observed in vitro is not the mechanism of γ formation in vivo.]