Evolution of Tn4371 family ICE; traR mediated coordination of cargo gene upregulation and horizontal transfer

ICEKKS102Tn4677, which has been shown to transfer horizontally, carries bph operon for mineralization of PCBs/biphenyl and belongs to an ICE Tn4371 family. In this study we investigated the role of traR gene encoding a LysR-type transcriptional regulator, which is conserved in sequence, positioning, and directional orientation among Tn4371 family ICEs. The traR belonged to bph operon and its overexpression on solid medium resulted in modest upregulation of traG (3-fold) and marked upregulation of xis (80-fold), and enhanced ICE excision, and notably ICE transfer frequency. We propose the evolutional roles of traR, which upon insertion to the current position, connected the cargo gene activation and ICE-transfer. This property of ICE, transferring under environmental conditions that lead to cargo gene activation, would give fitness advantages to the host bacteria, thereby resulting in efficient dissemination of the Tn4371 family ICEs. SignificanceOnly ICEKKS102Tn4677 is proven to transfer among the widely disseminating Tn4371 family ICEs from {beta} and {gamma}-proteobacteria. We showed that the traR gene in ICEKKS102Tn4677 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 ICE Tn4371 family ICEs. Our findings provide insights into the evolutionary processes that led to the widespread distribution of the Tn4371 family of ICEs across bacterial species.