Integrative and conjugative elements are bacterial mobile genetic elements that primarily reside in the host cell’s chromosome, yet have the ability to be transferred between cells by conjugation. ICEs can be considered as mosaic elements, combining features from other mobile elements: the integrative ability of bacteriophage or transposons, and the transfer mechanisms of conjugative plasmids. This mosaicism is reflected in modular structures: genes encoding the core functions of integration/excision, conjugation and regulation are generally found clustered together. As well as these core functions, ICEs often carry accessory genes that can bestow adaptive phenotypes on their hosts. Gene cassettes encoding antibiotic resistance, nitrogen fixation, virulence factors and various other functions have all been documented in ICEs. They are therefore important vectors for the horizontal dissemination of genetic information, facilitating rapid bacterial evolution.
Chromosomal integration and excision of ICEs is mediated by integrase (Int) enzymes. Most commonly integrases are tyrosine recombinases related to the well studied phage λ Int. They mediate site-specific recombination events between identical or near-identical sequences in the host and ICE genomes (termed attB and attP respectively). These integration events normally occur into tRNA genes. No definitive reason for this association of tyrosine recombinase mediated integration with tRNA genes is known, however tRNA genes evolve more slowly than protein coding genes, potentially broadening the possible host range. Other ICEs encode transposase family tyrosine recombinase Ints that have broader target sequence preferences. Members of the DDE transposase and serine recombinase families also serve as integrases in some ICEs. Before ICE transfer occurs, the element is excised and circularised. Excision of ICEs also requires Int activity, however the process is biased towards excision by ‘recombination directionality factors’ (RDFs). If chromosomal replication or cell devision occurs whilst the ICE is in the excised chromosomal state, the element could be lost from the cell. To prevent this ICEs (like plasmids) often encode addiction modules (toxin-antitoxin systems) that kill cells not inheriting the ICE.
Conjugative transfer occurs via the formation of a multiprotein apparatus that connects the donor and recipient cells: a type IV secretion system (T4SS). This consists of a membrane spanning secretion channel and often an extracellular pilus. The extrachromosomal ICE DNA is first nicked at the origin of transfer (oriT) by a relaxase (MOB) enzyme. Rolling circle replication is then initiated. MOB remains bound to the displaced single-stranded DNA and this nucleoprotein complex is targeted to the T4SS by a coupling protein (T4CP). Rather than using a T4SS, some ICEs are transferred between cells using FtsK-like DNA translocase pumps (in this case dsDNA is transferred). After transfer, the ssDNA ICE is replicated into dsDNA and integrated into the recipient cell’s chromosome. The ICE in the donor cell is also converted into dsDNA and re-integrated into the genome.
ICE transmission is under the control of networks responsive to environmental stimuli. For instance, transfer of the SXT-R391 family of ICEs is controlled by SelR, a homologue of the λ repressor CI. Regulation occurs by a similar mechanism as that controlling the λ switch from lysogeny to lysis. As with CI, SetR repression can be relieved by the action of RecA, the main effector of the ‘SOS’ response to DNA damage. Other ICEs transmission have been shown to be under the control of quorum sensing networks.
A recent bioinformatic study of ICE prevalence and diversity identified ICEs by finding clustered conjugative apparatus modules (Guglielmini et al). If these were found on chromosomal locations they were defined as belonging to ICEs, whilst those on extra-chromosomal elements were considered conjugative plasmids. No reference was made to the presence of integrases. Within this definition ICEs were more common than conjugative plasmids: 18% of sequenced prokaryotic genomes contained at least one ICE as opposed to 12% possessing conjugative plasmids. ICEs are generally defined as not being capable of autonomous extra-chromosomal replication and maintenance. This is opposed to conjugative plasmids that include replication origins and systems. However, this definition is not watertight, as there appear to be various exceptions. Likewise, conjugative plasmids can be integrated chromosomally, either by homologous recombination at repeat sequences, or by site-specific recombination events. These elements therefore exist on a spectrum. Phylogenetic analysis of VirB4 genes (an ATPase component of T4SS) shows that ICEs and conjugative plasmids do not segregate as monophyletic clades. Instead they are intermingled throughout the tree, suggesting that conjugative plasmids often become ICEs and vice versa. Guglielmini et al. therefore consider them as two sides of the same coin. If selection pressures are strong enough though, ICEs can be stabilised as chromosomal structures for long periods of time.
A striking example of the potency and evolutionary importance of ICEs is found in the genomes of the obligate intracellular bacterial family Rickettsiales. One third of the genome of Orientia tsutsugamushi (the mite borne causative agent of scrub typhus) is made up of degenerate copies of an ICE named RAGE (Rickettsiales amplified genetic element). Multiple invasions of RAGE have also configured the genome of a Rickettsial endosymbiont of a deer tick (REIS). In this case RAGEs have acted as hotspots for recombination and the insertion of other mobile elements, leading to the insertion of clusters of novel horizontally transferred genes (a process termed piggybacking). These two Rickettsiales species have especially large genomes for obligate intracellular bacteria, but it seems likely that RAGE has been important in the evolution of this entire clade.
See also: Expanding the Conjugative Realm
Wozniak, R., & Waldor, M. (2010). Integrative and conjugative elements: mosaic mobile genetic elements enabling dynamic lateral gene flow Nature Reviews Microbiology, 8 (8), 552-563 DOI: 10.1038/nrmicro2382
Guglielmini, J., Quintais, L., Garcillán-Barcia, M., de la Cruz, F., & Rocha, E. (2011). The Repertoire of ICE in Prokaryotes Underscores the Unity, Diversity, and Ubiquity of Conjugation PLoS Genetics, 7 (8) DOI: 10.1371/journal.pgen.1002222
Nakayama, K., Yamashita, A., Kurokawa, K., Morimoto, T., Ogawa, M., Fukuhara, M., Urakami, H., Ohnishi, M., Uchiyama, I., Ogura, Y., Ooka, T., Oshima, K., Tamura, A., Hattori, M., & Hayashi, T. (2008). The Whole-genome Sequencing of the Obligate Intracellular Bacterium Orientia tsutsugamushi Revealed Massive Gene Amplification During Reductive Genome Evolution DNA Research, 15 (4), 185-199 DOI: 10.1093/dnares/dsn011
Gillespie, J., Joardar, V., Williams, K., Driscoll, T., Hostetler, J., Nordberg, E., Shukla, M., Walenz, B., Hill, C., Nene, V., Azad, A., Sobral, B., & Caler, E. (2011). A Rickettsia Genome Overrun by Mobile Genetic Elements Provides Insight into the Acquisition of Genes Characteristic of an Obligate Intracellular Lifestyle Journal of Bacteriology, 194 (2), 376-394 DOI: 10.1128/JB.06244-11