Timing of integration into the chromosome is critical for the fitness of an integrative and conjugative element and its bacterial host

Abstract Integrative and conjugative elements (ICEs) are major contributors to genome plasticity in bacteria. ICEs reside integrated in the chromosome of a host bacterium and are passively propagated during chromosome replication and cell division. When activated, ICEs excise from the chromosome and may be transferred through the ICE-encoded conjugation machinery into a recipient cell. Integration into the chromosome of the new host generates a stable transconjugant. Although integration into the chromosome of a new host is critical for the stable acquisition of ICEs, few studies have directly investigated the molecular events that occur in recipient cells during generation of a stable transconjugant. We found that integration of ICE Bs1 , an ICE of Bacillus subtilis , occurred several generations after initial transfer to a new host. Premature integration in new hosts led to cell death and hence decreased fitness of the ICE and transconjugants. Host lethality due to premature integration was caused by rolling circle replication that initiated in the integrated ICE Bs1 and extended into the host chromosome, resulting in catastrophic genome instability. Our results demonstrate that the timing of integration of an ICE is linked to cessation of autonomous replication of the ICE, and that perturbing this linkage leads to a decrease in ICE and host fitness due to a loss of viability of transconjugants. Linking integration to cessation of autonomous replication appears to be a conserved regulatory scheme for mobile genetic elements that both replicate and integrate into the chromosome of their host. Author Summary Horizontal gene transfer helps drive microbial evolution, enabling bacteria to rapidly acquire new genes and traits. Integrative and conjugative elements (ICEs) are mobile genetic elements that reside integrated in the chromosome of a host bacterium and can transfer to other cells via a contact-dependent mechanism called conjugation. Some ICEs contain genes that confer important traits to their bacterial hosts, including antibiotic resistances, metabolic capabilities, and pathogenicity. Central to the propagation of ICEs and other mobile genetic elements is the balance between dissemination to new hosts and maintenance within a host, while minimizing the fitness burden imposed on their hosts. We describe an underlying regulatory mechanism that allows for an ICE that has entered into a nascent host to replicate and potentially spread to other hosts before integration into the chromosome. Integration occurs shortly after or concomitant with cessation of ICE replication. Disruption of this regulatory link results in premature integration and fitness defects for both the host and the ICE; lethality for the host and reduced spread of the ICE.