The HipAB Toxin-Antitoxin System Stabilizes a Composite Genomic Island in CN-32.

Composite genomic islands (GIs) are useful models for studying GI evolution if they can revert into the previous components. In this study, CGI48-a 48,135-bp native composite GI that carries GI21, whose homologies specifically integrated in the conserved gene-were identified in CN-32. CGI48 was integrated into the tRNA gene, which is a conserved gene locus for the integration of genomic islands in . Upon expressing integrase and excisionase, CGI48 and GI21 are excised from chromosomes site-specific recombination. The shorter attachment sites of GI21 facilitated the capture of GI21 into CGI48. Moreover, GI21 encodes a functional HipAB toxin-antitoxin system, thus contributing to the maintenance of CGI48 in the host bacteria. This study provides new insights into GI evolution by performing the excision process of the inserting GI and improves our understanding of the maintenance mechanisms of composite GI.