Evolution of the Tn4371 ICE family: traR-mediated coordination of cargo gene upregulation and horizontal transfer.

Abstract

ICEKKS102Tn4677 carries a bph operon for the mineralization of polychlorinated biphenyls (PCBs)/biphenyl and belongs to the Tn4371 ICE (integrative and conjugative element) family. In this study, we investigated the role of the traR gene in ICE transfer. The traR gene encodes a LysR-type transcriptional regulator, which is conserved in sequence, positioning, and directional orientation among Tn4371 family ICEs. The traR belongs to the bph operon, and its overexpression on solid medium resulted in modest upregulation of traG (threefold), marked upregulation of xis (80-fold), enhanced ICE excision and, most notably, ICE transfer frequency. We propose the evolutional roles of traR, 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 Tn4371 family ICEs.IMPORTANCEOnly ICEKKS102Tn4677 is proven to transfer among the widely disseminating Tn4371 family integrative and conjugative elements (ICEs) from β and γ-proteobacteria. We showed that the traR gene in ICEKKS102Tn4677, 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 traR by an ancestral ICE to the current position established the Tn4371 family of ICEs. Our findings provide insights into the evolutionary processes that led to the widespread distribution of the Tn4371 family of ICEs across bacterial species.