# Unbridled Integons: A Matter of Host Factors
## 6 Evolutionary Success of MI: A Matter of Host Factors
### The Evolutionary Success of MI Could Be Linked to a Specific Host Factor Independence  
It is tempting to speculate that the success of class 1 MI, carried on broad host-range plasmids and operational in multiple hosts, could be due to an IntI1 activity independent of specific host factors and dependent only on universal clues (Figure 8). This could explain why IntI1 is able to recombine at the same rate in _E. coli_ and _V. cholerae_ (see above). Classes 2 and 3 MI have also been shown to be functional in _E. coli_[98], and one can consider that the HF independence of class 1 MI holds true for these as well. This differential HF recruitment between MI and SCI is consistent with the observed widespread MI dissemination among bacterial species and their evolutionary success. It has been proposed that \(attI\) sites have co-evolved with their cognate integrases since cross recombination assays between non-cognate \(attI/IntI\) partners are generally inefficient. We suggest that the success of class 1 MI dissemination could be the result of this co-evolution between the integrase IntI1 and its cognate \(attII\) site, allowing cassette recombination to take place in absence of specific host factors. The high level of divergence between IntIA\({}_{Vch}\) and IntI1 (only 45% of identity [9; 13]) and the lack of identity between \(attIA\) and \(attII\) sites likely reflect the observed differences in their functioning. Indeed, \(attII\) contains two supplementary binding sites, the direct repeats (DR1 and DR2), while the \(attIA\) site does not present such repeated sequences. While these DR are dispensable for the \(attII\times attII\) reaction, it has been shown that their presence favors the \(attII\times attC\) reactions [99]. DR of \(attII\) sites may have been acquired and used as topological filters [99], thus replacing host factors that regulate supercoiling. However, dissemination of MI seems limited to the Gram-negative sphere. Indeed, only a few MI have been found in Gram positive bacteria [100; 101]. HF are hypothesized to impact integron ability to disseminate between distantly related species, for example, causing the important distribution bias between Gram negative and Gram-positive bacteria. This is surprising since both types can share the same environment and susceptibility to some antibiotics, an ideal situation for integron expansion. As mobile elements can be exchanged between them [102], MI would supposedly be able to spread in both groups. The yet unknown HF necessary for integron functioning could be involved in this discrepancy. In Gram positive bacteria, these HF would be too divergent from their functional counterparts (or even be simply absent) to allow integron functioning. The integron system would then be dormant, and its presence would be more costly than beneficial. Integon spread would thus be counter-selected, leading to this biased distribution.  
Figure 8: Evolution of integrons toward a limited host factor dependence. The figure shows the evolution of integrases followed by the capture of sedentary chromosomal integrons (SCI) by transposons and their mobilization on conjugative plasmids. The captured SCI are transferred by conjugation in bacteria and now called Mobile Integrons (MI). Their maintenance and dissemination in new bacteria reside in their capacity to have evolved limiting their specific host factor (HF) dependence.