Positioning ribosomes on leader mRNA for translational activation of the message of an inducible Staphylococcus aureus cat gene

The expression of the chloramphenicol (Cm) -inducible Cm acetyltransferase gene (cat) of the staphylococcal plasmid pUB112 is regulated at the translational level. The leader mRNA preceding the cat coding sequence can form a stable hairpin structure, in which the cat Shine-Dalgarno sequence is masked. Previous work showed that translation of a short leader peptide terminating within the stem of the inhibitory secondary structure is required for basal Cm acetyltransferase (CAT) synthesis and its inducibility. In the present study we shortened this leader peptide by introducing ochre codons in its coding sequence and found that synthesis of the N-terminal part of the leader peptide, terminating directly 5′ to the stem, is sufficient to mediate basal and inducible CAT synthesis. Amino acid substitution in this region of the leader peptide abolished inducibility. We suggest that the 5′ region of the leader peptide coding sequence specifies a particularly Cm-sensitive translation that represents the Cm-sensor mechanism for cat gene induction.