Transposition mechanism of IS Apl1 -the determinant of colistin resistance dissemination.

Multidrug-resistant Enterobacteriaceae , a prominent family of gram-negative pathogenic bacteria, causes a wide range of severe diseases. Strains carrying the mobile colistin resistance ( mcr-1 ) gene show resistance to polymyxin, the last line of defense against multidrug-resistant gram-negative bacteria. However, the transmission of mcr-1 is not well understood. In this study, genomes of mcr-1 -positive strains were obtained from the NCBI database, revealing their widespread distribution in China. We also showed that IS Apl1 , a crucial factor in mcr-1 transmission, is capable of self-transposition. Moreover, the self-cyclization of IS Apl1 is mediated by its own encoded transposase. The electrophoretic mobility shift assay experiment validated that the transposase can bind to the inverted repeats (IRs) on both ends, facilitating the cyclization of IS Apl1 . Through knockout or shortening of IRs at both ends of IS Apl1 , we demonstrated that the cyclization of IS Apl1 is dependent on the sequences of the IRs at both ends. Simultaneously, altering the ATCG content of the bases at both ends of IS Apl1 can impact the excision rate by modifying the binding ability between IRs and ISAPL1. Finally, we showed that heat-unstable nucleoid protein (HU) can inhibit IS Apl1 transposition by binding to the IRs and preventing ISAPL1 binding and expression. In conclusion, the regulation of IS Apl1 -self-circling is predominantly controlled by the inverted repeat (IR) sequence and the HU protein. This molecular mechanism deepens our comprehension of mcr-1 dissemination.