Abstract
Induction of chloramphenicol (Cm) acetyltransferase-encoding genes (
cat) by Cm has been proposed to result from the destabilization of a stem-loop that sequesters the ribosome-binding site for the
cat coding sequence. Destabilization is caused by the stalling of a ribosome at a specific site in the leader of
cat transcripts that immediately precedes the stem-loop. By use of in vivo dimethylsulfate probing of
cat-86 leader mRNA, we demonstrate the existence of the stemloop structure in
cat transcripts isolated from uninduced cells and its release during induction. Leader mutations chosen to provide the mRNA with an alternative folding pattern that destabilizes the stem-loop cause constitutive
cat expression. Our results establish the occurrence in vivo of the stem-loop in
cat-86 transcripts and its role as a negative regulator of
cat expression.