Evolution of the Tn ICE family: -mediated coordination of cargo gene upregulation and horizontal transfer.

ICETn carries a operon for the mineralization of polychlorinated biphenyls (PCBs)/biphenyl and belongs to the Tn ICE (integrative and conjugative element) family. In this study, we investigated the role of the gene in ICE transfer. The gene encodes a LysR-type transcriptional regulator, which is conserved in sequence, positioning, and directional orientation among Tn family ICEs. The belongs to the operon, and its overexpression on solid medium resulted in modest upregulation of (threefold), marked upregulation of (80-fold), enhanced ICE excision and, most notably, ICE transfer frequency. We propose the evolutional roles of , which upon insertion to its current position, might have connected the cargo gene activation and ICE transfer. This property of ICE, i.e., undergoing transfer under environmental conditions that lead to cargo gene activation, would instantly confer fitness advantages to bacteria newly acquiring this ICE, thereby resulting in efficient dissemination of the Tn family ICEs.IMPORTANCEOnly ICETn is proven to transfer among the widely disseminating Tn family integrative and conjugative elements (ICEs) from β and γ-proteobacteria. We showed that the gene in ICETn, which is conserved in the ICE family with fixed location and direction, is co-transcribed with the cargo gene and activates ICE transfer. We propose that capturing of by an ancestral ICE to the current position established the Tn family of ICEs. Our findings provide insights into the evolutionary processes that led to the widespread distribution of the Tn family of ICEs across bacterial species.