Class 1 integrons in Acinetobacter baumannii: a weak expression of gene cassettes to counterbalance the lack of LexA-driven integrase repression.

Abstract Integrons recruit resistance genes through integrase-driven recombination events that are regulated by the bacterial SOS response and require the repressor LexA. Class 1 integrons genes are expressed from a common promoter, Pc, of which at least 5 predominant variants, classified from weak to strong, have been described. In Escherichia coli , there is an intertwined regulation between gene cassette expression and integrase activity: the stronger the promoter, the weaker the integrase. Class 1 integrons have been frequently described in Acinetobacter baumannii . However, Acinetobacter spp. lack the LexA repressor, suggesting that the integrase is constitutively expressed. We characterized the integron content of 83 clinical and environmental A. baumannii strains. We found a predominance of Pc variants described as strong in E. coli . The Pc expression level was 2- to 4-fold lower in A. baumannii than in E. coli , and the diversity of the gene cassette array was low. In A. baumannii , integrons with a PcS promoter might have been selected to enable sufficient resistance while avoiding the toxicity of a highly active integrase. Furthermore, a transcriptional interference between PcS and PintI1 (as shown in E. coli ) may limit the expression of the integrase and thus counterbalance the lack of LexA-driven integrase repression to prevent the cost of the integrase.