Integrative and conjugative element
Integrative and conjugative elements (ICEs) are mobile genetic elements present in both gram-positive and gram-negative bacteria. In a donor cell, ICEs are located primarily on the chromosome, but have the ability to excise themselves from the genome and transfer to recipient cells via bacterial conjugation.
Due to their physical association with chromosomes, identifying integrative and conjugative elements has proven challenging, but in silico analysis of bacterial genomes indicate these elements are widespread among many microorganisms.[1][2]
ICEs have been detected in Pseudomonadota (e.g., Pseudomonas spp., Aeromonas spp., E. coli, Haemophilus spp.), Actinomycetota and Bacillota. Among many other virulence determinants, ICEs may spread antibiotic and metal ion resistance genes across prokaryotic phyla.[1][3][4][5] In addition, ICE elements may also facilitate the mobilisation of other DNA modules such as genomic islands.[3][6]
Characteristics
Although ICEs exhibit various mechanisms promoting their integration, transfer and regulation, they share many common characteristics. ICEs comprise all mobile genetic elements with self-replication, integration, and conjugation abilities, including conjugative transposons, regardless of the particular conjugation and integration mechanism by which they act. Some immobile genomic pathogenicity islands are also believed to be defective ICEs that have lost their ability to conjugate.
ICEs combine certain features of the following mobile genetic elements:[1]
- Bacteriophagesthat have the ability to insert into and excise from bacterial chromosomes.
- Transposons that, besides their inherent transposable activity, can additionally be subject to horizontal gene transfervia conjugation.
- Conjugative plasmidsthat transfer from donor to recipient bacteria via conjugation.
In contrast to plasmids and phages, integrative and conjugative elements cannot remain in an extrachromosomal form in the cytoplasm of bacterial cells and replicate only with the chromosome they reside in.
ICEs possess the structure organized into three gene modules that are responsible for their integration with the chromosome, excision from the genome and conjugation, as well as regulatory genes.