## 2 Transcriptional Regulation in Integrons
### The Role of Pc Promoters on Gene Cassette Transcription
#### 2.1.1 Pc Promoters in Class 1 Integrons
More recent studies revealed that PcW + P2, PcW\({}_{\text{TGN-10}}\), and PcW\({}_{\text{TGN-10}}\) + P2 variants occurred in PcW-containing integrins much more frequently than PcW alone [26; 29; 34]. The active P2 had also been rarely identified in association with PcH1 and PcS [28]. Additionally, three rare mutated forms of active P2, named P2m1, P2m2, and P2m3, were reported in association with PcW and PcH1 [26; 29]. A fourth P2 variant, named here P2m4, was found in a defective integrin harboring a truncated integrase gene. This promoter presented a polymorphism in the first position of -10 hexamer relative to the canonical P2 (Table 2). This truncation in _intI1_ led to Pc deletion, in a way that P2m4 was the unique common promoter in that integer and seemed to control gene cassette expression [41]. So far, dozens of these Pc variants and Pc + P2 combinations had their activity experimentally determined (Table 2). It was revealed distinct relative transcription strengths that directly affect the expression of resistance gene cassettes inserted in class 1 integrins and, consequently, the level of antibiotic resistance. Several groups have performed such studies on Pc promoter activity in different moments and using different approaches [2; 3; 26; 28; 29; 38; 42; 43], what limited the comparison of the results. [PERSON] and Colleagues compiled all Pc variants described until that occasion, including those for which transcription activity was unknown and new variants identified in that study, and determined their transcription efficiencies under the same conditions [26], as detailed in Table 2. This study demonstrated that the relative transcription strengths of known Pc variants corroborate those obtained in previous studies [2; 3; 28], in which PcS was the strongest promoter, followed by PcH2, PcW + P2, PcH1, and PcW. Considering the newly identified Pc variants, PcIn116 presented the lowest transcription efficiency, while PcPUO and PcHn42 strengths were similar to that of PcW [26]. Although PcS had been previously described as a promoter stronger than PcS [38], [PERSON] and Colleagues demonstrated that its transcription was about 12-fold less efficient than that driven by PcS. The TGN-10 motif provided a significant increase in PcW efficiency, approaching that of PcH2 and two-fold higher than that of PcW + P2, while no relevant effect on the activity of the strongest PcS and PcH2 was observed in the presence of the TGN-10 motif. Conversely, the TAN-10 motif in the PcW\({}_{\text{TAN-10}}\) configuration, severely reduced the PcW activity, while the TTN-10 motif found in the PcH1\({}_{\text{TNN-10}}\) version, slightly increased promoter efficiency similar to that found for PcH1 + P2 (Table 2) [26]. The P2 promoter contributed to a remarkable increase in transcription when associated with PcW and PcH1 versions, however, when combined with the strongest PcS and PcW\({}_{\text{TGN-10}}\) variants, their efficiencies suffered a slight decrease. The newly mutated forms of P2, P2m1, and P2m2 seemed to be inactive [26], however, the P2m3 promoter, found in association with PcW, was able to double promoter strength, approaching the PcW\({}_{\text{TGN-10}}\) expression level (Table 2) [29]. A recent study demonstrated the role of PcH1, PcS, and PcW\({}_{\text{TGN-10}}\) on the expression of a variety of resistance gene cassette arrays in class 1 integron-positive _Proteus_ strains [44]. All of them were able to drive cassette expression, and strains containing stronger promoters, such as PcS and PcW\({}_{\text{TGN-10}}\), presented higher antibiotic resistance rates than strains with weaker promoters (PcH1).