The hidden life of integrative and conjugative elements

Abstract

Integrative and conjugative elements (ICEs) are widespread mobile DNA that transmit both vertically, in a host-integrated state, and horizontally, through excision and transfer to new recipients. Different families of ICEs have been discovered with more or less restricted host ranges, which operate by similar mechanisms but differ in regulatory networks, evolutionary origin and the types of variable genes they contribute to the host. Based on reviewing recent experimental data, we propose a general model of ICE life style that explains the transition between vertical and horizontal transmission as a result of a bistable decision in the ICE–host partnership. In the large majority of cells, the ICE remains silent and integrated, but hidden at low to very low frequencies in the population specialized host cells appear in which the ICE starts its process of horizontal transmission. This bistable process leads to host cell differentiation, ICE excision and transfer, when suitable recipients are present. The ratio of ICE bistability (i.e. ratio of horizontal to vertical transmission) is the outcome of a balance between fitness costs imposed by the ICE horizontal transmission process on the host cell, and selection for ICE distribution (i.e. ICE ‘fitness’). From this emerges a picture of ICEs as elements that have adapted to a mostly confined life style within their host, but with a very effective and dynamic transfer from a subpopulation of dedicated cells.

Salmonellagenomic island 3 is an integrative and conjugative element and contributes to copper and arsenic resistance ofSalmonella enterica

“…In this study, we performed the conjugation experiment without any chemicals inducing the SOS response. The addition of mitomycin C or antimicrobials might leads to successful conjugative transfer of SGI3 to these strains (19).…”

“…Integrative and conjugative elements (ICEs) are self-transmissible between chromosomes of different cells (17, 18) and are widely distributed among both Gram-negative and Gram-positive bacteria (19). ICEs include genes defining a type IV secretion system, which contributes to conjugative transfer, in addition to an integrase and an excisionase (17, 19). ICEs can excise themselves from the donor chromosome, form a circular intermediate, transfer by conjugation and reintegrate into the recipient chromosome.…”

Genome sequence ofPseudomonas aeruginosaPAO1161, a PAO1 derivative with the ICEFP2 integrative and conjugative element

“…ICEs are mobile genetic elements, with a modular structure, encoding complete conjugation machinery (usually a type IV secretion system) allowing transfer of their genome to another host. They are reversibly integrated into a host genome and can be passively propagated during bacterial chromosome segregation and cell division [62][63][64] . PAPI-1 (108 kb, 115 orfs, integrated in tRNA Lys ) was first described in the genome of highly virulent P. aeruginosa PA14 strain 65 .…”

A four-step regulatory cascade controls bistable transfer competence development of the integrative and conjugative element ICEclcinPseudomonas

“…1A) [9]. In the majority of cells ICE clc is maintained in the integrated state, whereas a small proportion of cells (3-5%) in stationary phase activates the ICE transfer competence program [9–11]. Upon resuming growth, transfer competent (tc) donor cells excise and replicate the ICE [12], which can conjugate to a recipient cell, where the ICE can integrate [10].…”