Fig. 4. Frameshifting in modification-deficient mutants. Six NNA mutants (three low frameshifters and three high) were introduced into isogenic wild-type, aroD, mnmA and mnmE strains. Frameshifting was estimated by measuring the β-galactosidase activity of the resulting strains (in the absence of IPTG). The construct in which g10 and lacZ are in the same frame (in-phase construct in Figure 1) served to define the 100% reference activity and a construct in which the A AAG sequence was mutated to a non-slippery one (G AAA) was used to determine the background level.