Constitutive expression of cat-86 associated with a change in the transcription start point.

The translational attenuation regulatory model suggests a mechanism that can explain the induction of cat-86 by chloramphenicol (Cm). In this model, Cm serves to stall a ribosome at a specific site in a leader region of cat-86 transcripts. The stalled ribosome is thought to destabilize a downstream region of RNA secondary structure that normally sequesters the cat-86 ribosome-binding site (RBS-3). Three mutations in codon 4 of the cat-86 leader have been identified which result in constitutive cat expression. Each of the three mutations generates a likely -10 promoter sequence in the leader. Twenty nucleotides (nt) upstream is the wild-type sequence, 5'-TTGAAA, which differs from the consensus sigA -35 domain by only a single nt. The transcription start point from the resulting mutant promoter is within the DNA region that normally specifies the RNA secondary structure that sequesters cat-86 RBS-3. Thus, the basis for the constitutive phenotype is the absence of the RNA secondary structure in the transcripts driven by the promoter generated through mutagenesis of leader codon 4.