Four-base codons ACCA, ACCU and ACCC are recognized by frameshift suppressor sufJ.

The frameshift suppressor sufJ acts to correct a set of +1 frameshift mutations having very different sequences at their mutant sites. This suppressor acts by reading a 4 base codon located near, but not at, the site of each suppressible mutation. Suppression thus necessitates out-of-phase translation of the short stretch of mRNA between the site of action of the suppressor tRNA and the site of the frameshift mutation. We have identified the site read by sufJ by mutationally creating a series of such sites in the neighborhood of a previously nonsuppressible frameshift mutation. Each of the newly generated sites was formed by base substitution. Four independently generated sites were analyzed by DNA sequencing. At each site the quadruplet codon ACCX was generated (where X is A, U or C). Thus sufJ is able to read a 4 base codon in which any of three bases is acceptable in the fourth position. This is the first frameshift suppressor that does not read a run of three repeated bases in the first three positions of its codon.