Transcription of integron-harboured gene cassette impacts integration efficiency in class 1 integron

Class 1 integrons play important roles in the dissemination of antibiotic resistance genes among bacteria. Generally, class 1 integron consists of an integrase gene (intI1), a recombination site (attI1) and a promoter (Pc) that drives the transcription of the downstreamed gene cassettes. Occasionally, there is a second promoter P2 downstream of the Pc promoter. Several Pc variants and Pc-P2 combinations have been defined and they display different transcription strengths, but the influence of the transcription of integron-harboured gene cassette on the integration efficiency has never been comprehensively studied. In this study, the integration frequencies of gene cassettes into the attI1 sites that downstream of four different Pc variants as well as their combinations with P2 promoter were measured. The results showed that there was an inverse correlation between the strength of Pc promoter and the integration efficiency and, with the same Pc promoter, the integration efficiency was significantly decreased when a P2 promoter preceded the attI1 site. Our findings indicate there is a relationship between the transcription of integron-harboured gene cassette and the integration of exogenous gene cassettes. The interrelationship between these two relatively independent processes may throw a light on our understanding the regulation system of class 1 integron.