Integrative and conjugative elements: mosaic mobile genetic elements enabling dynamic lateral gene flow

Key Points Integrative and conjugative elements (ICEs) are found in a diverse array of Gram-negative and Gram-positive bacteria. ICEs are integrated into host chromosomes but can excise, circularize and transfer (through conjugation) to neighbouring cells. The genes encoding key components of the ICE life cycle are often grouped into functional modules. Modules may be exchanged among ICEs as well as with other mobile elements that comprise the mobilome. In addition to the core modules that mediate ICE integration, excision, conjugation and regulation, ICEs routinely encode a range of accessory functions, including virulence factors and resistance proteins for antibiotic and heavy metal resistance. ICEs integrate with varying degrees of site specificity. Integrases, which mediate integration, are typically tyrosine recombinases, although there are a few cases of ICEs using a DDE transposase or a serine recombinase for this function. Integrases are also required for excision, although other factors are usually required in addition. Conjugal transfer requires DNA processing, which is accomplished by a relaxase. Rolling circle replication is thought to be the primary process that liberates a single-stranded DNA molecule for transfer. The type IV secretion system seems to be the most common mechanism used by ICEs for horizontal DNA transfer. There are varied and complex mechanisms that govern ICE transfer. Many ICEs encode unique factors that influence their excision and transfer frequencies. Integrative and conjugative elements are self-transmissible genetic elements that encode a core set of genes for their propagation and transfer and can also carry various other genes, including drug resistance markers. Here, Wozniak and Waldor describe the transmission of these elements and how this is regulated. Integrative and conjugative elements (ICEs) are a diverse group of mobile genetic elements found in both Gram-positive and Gram-negative bacteria. These elements primarily reside in a host chromosome but retain the ability to excise and to transfer by conjugation. Although ICEs use a range of mechanisms to promote their core functions of integration, excision, transfer and regulation, there are common features that unify the group. This Review compares and contrasts the core functions for some of the well-studied ICEs and discusses them in the broader context of mobile-element and genome evolution.