Biology of ICEBs1, an integrative and conjugative element in Bacillus subtilis

• ICEBs1 is an integrative and conjugative element in Bacillus subtilis.
• This review describes the life cycle of ICEBs1, including excision and transfer.
• Other aspects of ICEBs1 biology are described such as its replication and immunity.
• ICEBs1 gene expression is regulated by DNA damage and cell density.
• A new model for the ICEBs1 DNA translocation channel is presented.

Horizontal gene transfer plays a profound role in bacterial evolution by propelling the rapid transfer of genes and gene cassettes. Integrative and conjugative elements (ICEs) are one important mechanism driving horizontal gene transfer. ICEs, also known as conjugative transposons, reside on the host chromosome but can excise to form a conjugative DNA circle that is capable of transfer to other cells. Analysis of the large number of completed bacterial genome sequences has revealed many previously unrecognized ICEs, including ICEBs1, found in the Gram-positive model bacterium Bacillus subtilis. The discovery of ICEBs1 in an organism with such an impressive array of molecular tools for genetics and molecular biology was fortuitous. Significant insights into ICE biology have resulted since its discovery < 15 years ago. In this review, we describe aspects of ICEBs1 biology, such as excision, conjugative transfer, and reintegration, likely to be conserved across many ICEs. We will also highlight some of the more unexpected aspects of ICEBs1 biology, such as its ability to undergo plasmid-like replication after excision and its ability to mobilize plasmids lacking dedicated mobilization functions. A molecular understanding of ICEBs1 has led to additional insights into signals and mechanisms that promote horizontal gene transfer and shape bacterial evolution.