Diversity and evolution of an abundant ICEclc family of integrative and conjugative elements in Pseudomonas aeruginosa.

IMPORTANCE: Microbial populations swiftly adapt to changing environments through horizontal gene transfer. While the mechanisms of gene transfer are well known, the impact of environmental conditions on the selection of transferred gene functions remains less clear. We investigated ICEs, specifically the ICEclc-type, in Pseudomonas aeruginosa clinical isolates. Our findings revealed co-evolution between ICEs and their hosts, with ICE transfers occurring within strains. Gene functions carried by ICEs are positively selected, including potential virulence factors and heavy metal resistance. Comparison to publicly available P. aeruginosa genomes unveiled widespread antibiotic-resistance determinants within ICEclc clades. Thus, the ubiquitous ICEclc family significantly contributes to P. aeruginosa's adaptation and fitness in diverse environments.

Characterization of an atypical but widespread type IV secretion system for transfer of the integrative and conjugative element (ICEclc) in Pseudomonas putida.

Conjugation of DNA relies on multicomponent protein complexes bridging two bacterial cytoplasmic compartments. Whereas plasmid conjugation systems have been well documented, those of integrative and conjugative elements (ICEs) have remained poorly studied. We characterize here the conjugation system of the ICEclc element in Pseudomonas putida UWC1 that is a model for a widely distributed family of ICEs. By in frame deletion and complementation, we show the importance on ICE transfer of 22 genes in a 20-kb conserved ICE region. Protein comparisons recognized seven homologs to plasmid type IV secretion system components, another six homologs to frequent accessory proteins, and the rest without detectable counterparts. Stationary phase imaging of P. putida ICEclc with in-frame fluorescent protein fusions to predicted type IV components showed transfer-competent cell subpopulations with multiple fluorescent foci, largely overlapping in dual-labeled subcomponents, which is suggestive for multiple conjugation complexes per cell. Cross-dependencies between subcomponents in ICE-type IV secretion system assembly were revealed by quantitative foci image analysis in a variety of ICEclc mutant backgrounds. In conclusion, the ICEclc family presents an evolutionary distinct type IV conjugative system with transfer competent cells specialized in efficient transfer.