Influence of class 2 integron integrase concentration on gene cassette insertion and excision in vivo

Integron can capture and express antimicrobial resistance gene cassettes and plays important roles in horizontal gene transfer. The establishment of a complete in vitro reaction system will help to reveal integron integrase mediated site-specific recombination process and regulation mechanism. As an enzymatic reaction, the concentration of integrase is assumed to have a great influence on the reaction rate. To determine the influence of different concentrations of integrase on the reaction rate and to find the best range of enzyme concentration were essential to optimizing the in vitro reaction system. In this study, plasmids with gradient transcription levels of class 2 integron integrase gene intI2 under different promoters were constructed. Among plasmids pI2W16, pINTI2N, pI2W, and pI2NW, intI2 transcription levels ranged from about 0.61-fold to 49.65-fold of that in pINTI2N. And the frequencies of gene cassette sat2 integration and excision catalyzed by IntI2 were positively correlated with the transcription levels of intI2 within this range. Western blotting results indicated high expression of IntI2 partly existed in the form of an inclusion body. When compared with Pc of class 1 integron, the spacer sequence of P-intI2 can increase the strength of PcW but decrease the strength of PcS. In conclusion, the frequencies of gene cassette integration and excision were positively correlated with the concentration of IntI2. intI2 driving by PcW with P-intI2 spacer sequence can obtain the optimum IntI2 concentration required to achieve the maximum recombination efficiency in vivo in this study.