Bacillus subtilis mutant allele sup-3 causes lysine insertion at ochre codons: use of sup-3 in studies of translational attenuation

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Abstract
The mutation sup-3 in Bacillus subtilis suppresses ochre (TAA) mutations at each of three codons in the 5' end of the cat-86 coding sequence. The suppressor is shown to insert lysine at ochre codons. The efficiency of suppression by sup-3 is about 15%, as determined by changing a cat-86 Lys codon (codon 12) to an ochre codon and measuring the level of CAT in the suppressor-containing strain. The results obtained are discussed in light of previous observations that ochre mutations at cat leader codons 2 and 3 can be phenotypically suppressed by sup-3, whereas ochre mutations at leader codons 4 and 5 cannot. Translation of the cat leader is essential to inducible expression of cat. Our data support the interpretation that the nature of amino acids 2 through 5 of the leader peptide contributes to determining whether chloramphenicol can stall a ribosome in the leader, which in turn leads to induction of cat expression.