Figure 1.
Map not to scale of the pKQV4-based constructs used in this work. Upon addition of the gratuitous inducer IPTG the Lac repressor, encoded by lacIq (open arrow) dissociates from the operator region Olac (gray box) and transcription initiates at promoter Ptac (bold arrow). Transcription terminates at the transcription terminator Trrnb (gray box). The transcription initiated at Ptac drives the expression of the lacZ gene (large open arrow) cloned between the EcoRI and HindIII sites (underlined). The used lacZ variants carried different codons in the +2 and +3 positions. Translation initiation occurs, thorough association of the Shine-Dalgarno (SD, bold) sequence in the mRNAs and the ribosomes, at the translation initiation codon ATG. The constructs were selected in transformed cells resistant to ampicillin conferred by the gene bla (open arrow), which encodes β-lactamase. The relative position of the plasmid replication origin (ori) is indicated. The segments in bold indicate other vector DNA sequences.