# Unbridled Integons: A Matter of Host Factors
## 4 Host Factors Influencing the Expression of the Integrase
### Integrase Expression and SOS Response
#### 4.1.3 SOS Response and Horizontal Gene Transfer  
Another important source of ssDNA in bacteria is horizontally transferred DNA. HGT is a primordial aspect of bacterial evolution. HGT allows adaptation at a much higher pace through parallelized accumulation of adaptive traits [50]. For integrons, combination of metagenomics and comparative genomics approaches suggests that there is a very large pool of existing integron cassettes that can be found in a virtually infinite range of environments [51]. The only way those sets of cassettes coming from various backgrounds can be effectively compiled together within individual integrons is through HGT. Interestingly, in two of the most prevalent mechanisms of HGT in bacteria, conjugation, and transformation, the DNA is acquired in form of ssDNA [52]. During conjugation, a relaxase creates a nick in one of the strands of the plasmid DNA at the oriT. The nicked strand is then unwound from the unbroken strand and transferred to the recipient cell, which leads to the transient occurrence of ssDNA in the recipient cell [53]. This process has been visualized in real time and in single cells of _Escherichia coli_[54]. This triggers the SOS response and consequently allows the integrase to be expressed in recipient cells, as was shown in _E. coli_ and _V. cholerae_[55]. One of the consequences is to induce cassette mobility within the host integro. In particular, in recipient _V. cholerae_ cells, cassette shuffling within the SCI can be observed at quite high rates [56]. Whether in a clinical setting or in the environment, conjugation happens in high cell density conditions such as in biofilms or within the intestinal tract in the case of _V. cholerae_. In such contexts, conjugation can occur between very distantly related species [57]. As a consequence, a given MI on a conjugative plasmid can be quickly mobilized and encounter a very wide range of genetic backgrounds. If the species encountered by such an MI contain a SCI, it is easy to imagine that the sole conjugation-induced expression of the MI integrase could be sufficient to recruit useful functions from the SCI to the MI. It was shown, for instance, that IntI1 induction in a strain of _V. cholerae_ containing the R388 conjugative plasmid could cause the recruitment of the _catB9_ cassette from the SCI to the MI present in R388 [58]. Subsequent conjugation of that plasmid conferred resistance to chloramphenicol to the recipient _E. coli_ strain. Hence, conjugation clearly is a highly relevant way to couple the mobility of integrons with their ability to confer extreme adaptability to their host. Transformation is another important route for HGT, although it is restricted to the species that can develop competence in some specific conditions. Upon induction of competence, the mechanism of DNA uptake through transformation is relatively conserved in the different naturally competent species [59]. The exogenous dsDNA is the substrate for transformation: one strand is degraded, and the other is internalized through the ComEC transmembrane channel. Again, the result is that the newly acquired DNA is in its single-stranded form, inducing the SOS response in the recipient cell [20]. There are more than 80 species known to be naturally competent; _V. cholerae_ is one of them, and likely most _Vibrio_ species as their genomes are found to carry the competence genes [59]. It was found that _V. cholerae_ acquires competence in presence of chitin, a component of the crustacean shells, which is coherent with the marine life-style of this species [60]. Similarly to what has been shown for conjugation, the chitin-induced uptake of exogenous DNA was sufficient to induce the expression of the integrase through the SOS [20]. In a recent report, for the first time, we provided evidences for cassette recruitment within the SCI of _V. cholerae_ while relying solely on the SOS-dependent induction of IntIA [56].