## 2 Transcriptional Regulation in Integrons
### Regulation of Integrase Transcription in MI and CI by Stress Response Mechanisms
#### 2.5.1 The Role of SOS Response
In fact, a previous study demonstrated that the antibiotic-induced SOS response led to the one-step emergence of the ceftazidime-resistant phenotype during the course of human infection, due to the re-ordering of a previously silenced gene cassette by an active class 1 integrase. They observed that the previous treatment of a patient with metronidazoleze and ceftazidime induced the SOS response, with the concomitant activation of integrase expression. This integrase  
Figure 8: Sequence and organization of the Pc, P2 and P\({}_{\text{intl}}\) promoters and the LexA binding site in class 1 integrin. The -35 and -10 hexamers of all promoters are boxed and the GGG insertion that creates the active P2 is underlined. The transcriptional initiation sites (TSS) are indicated in bold with broken arrows. The integrase start codon is shown in bold underlined uppercase letters. The overlapped region corresponding to transcripts originating from the inversely oriented Pc and P\({}_{\text{intl}}\) that leads to transcription interference is shaded in grey (**top**). The sequence and organization of the LexA binding site are highlighted by a dotted box (**bottom**). exised an upstream cassette allowing the proper and full expression of the _bla_OXA-28 gene, resulting in the manifestation of the observed ceftazidime resistance. Therefore, this study showed that metronidazole induction of the SOS response favored the emergence of highly resistant bacteria within a patient that could be disseminated under selective antibiotic pressure [104]. Since the SOS system is activated by the accumulation of ssDNA molecules, horizontal gene transfer events that lead to the transient formation of the exogenous ssDNA could also contribute to the induction of SOS response. In fact, in vitro, and in vivo studies revealed that the SOS response was induced in _E. coli_ and _V. cholerae_ cells during plasmid conjugative transfer leading to the upregulation of _intI_ and _intI_A expression, which, in turn, triggered cassette recombination in both MI and CI platforms [18]. Besides conjugation, ssDNA uptake by naturally competent cells also induces SOS system and integrase expression, demonstrating the role of transformation events in the regulation of integron functionality [21]. This intricate regulatory mechanism is also demonstrated by the significant correlation observed between the loss of LexA repression and the integrase inactivation, indicating that recombination events mediated by a constitutively expressed integrase may be deleterious to the host [19]. However, this lack of LexA regulation can be accepted or selected upon providing adaptive advantages. For example, it had already been demonstrated that the creation of a functional P2 promoter, by the insertion of a GGG triplet in its shortened spacer region, evoked heightened rates of resistance gene cassette expression in class 1 integrins harboring the weak PcW configuration [2; 3]. However, the GGG insertion  
Figure 9: Schematic representation of the interplay between the SOS response induction and the integron activity. The basic structure of integrons (integrase _intI_ gene, the _attI_ recombination site, and the Pc and P\({}_{\text{intI}}\) promoters) is indicated. The first cassettes expressed by the Pc promoter are indicated by arrows filled from faded black to light gray colors representing the most and least expressed cassettes, respectively, while distal silenced cassettes are represented by blank arrows. The mechanism of integrase expression regulation by SOS response is shown. SOS stress response inducers lead to LexA autoclaye, releasing the P\({}_{\text{intI}}\) promoter and derepressing integrase expression. Circular cassette intermediate resulted from excision (_attC X attC_ recombination) of distal positions and its possible reinsertion (_attI_ X attC_ recombination) next to Pc mediated by integrase activity upon derepression by SOS induction are demonstrated.