Frameshifting in the expression of the E. coli trpR gene occurs by the bypassing of a segment of its coding sequence

The E. coli trpR gene encodes the 108 amino acid long trp repressor. We have previously shown that a + 1 frameshifting event occurs during the expression of trpR. Here we show that the transition from the 0 to the + 1 frame of trpR occurs by the bypassing of a 55 nt long segment of the trpR₊₁-lacZ mRNA. This bypassing event is not pretranslational, and it probably takes place during translation. Two adjacent elements are required: a specific sequence of trpR, which must be preceded by a nonspecific 5′ end longer than 10 translatable codons. Unique to trpR-lacZ bypassing is that the 55 nt long region must be translated in frame 0 to enable bypassing into the + 1 frame. Translational bypassing as a newly discovered mechanism of gene expression is discussed, and the possible existence of translational introns is suggested.