Mobile ISCR elements: Structure, functions, and role in emergence, increase, and spread of blocks of bacterial multiple antibiotic resistance genes

The recently discovered method of horizontal distribution of bacterial genes with ISCR atypical insertion sequences is reviewed using the example of drug-resistance genes. The mechanism of transposition of such elements including rolling circle replication, formation of autonomous nonreplicable circular structures, and homologous recombination provides mobilization of any section of the adjacent DNA. ISCR elements represent a more powerful gene mobilization system than transposons and integrons and provide formation of groups of mobile genes, including antibiotic resistance genes of pathogenic bacteria. The structure and functions of ISCR elements are discussed together with their similarity and dissimilarity to IS91-like elements and their role in emergence of blocks of multiple antibiotic resistance genes and their contribution to evolution of bacteria and plasmids.