Autonomous plasmid-like replication of Bacillus ICEBs1: a general feature of integrative conjugative elements?

Integrative conjugative elements (ICEs) occur frequently in Gram-positive and Gram-negative bacteria. In contrast to plasmids, they are stably integrated in the bacterial genome, often inserted in a tRNA gene. They are excised from the host chromosome upon induction in order to be transferred to a recipient cell. When conjugative transfer is completed, they stably reintegrate in the chromosome. It is generally thought that ICEs are incapable of autonomous replication, instead relying on replication and segregation along with the host chromosome. In this issue of Molecular Microbiology Lee and co-workers demonstrate that ICEBs1 from Bacillus subtilis is capable of autonomous plasmid-like replication in its circular form after excision. The authors show that ICEBs1 replication is unidirectional; it initiates at oriT(ICEBs1) and requires the ICEBs1-encoded conjugative relaxase NicK. Replication also requires the catalytic subunit of the host DNA polymerase PolC, the host processivity clamp DnaN and the host-encoded alternative helicase PcrA. Autonomous replication of ICEBs1 appears to be important for its stable maintenance, but not for horizontal transfer of the element. Lee and co-workers argue that plasmid-like replication is likely a common property of ICEs, probably contributing to stability and maintenance of ICEs in bacterial populations. I discuss these findings in context with data on other ICEs from Gram-positive and Gram-negative bacteria and with respect to possible consequences of the findings for basic research on mobile genetic elements from Gram-positive bacteria and their applications in biotechnology.