⚡ Core Concept: The Mobilome: Plasmids, Transposons & Integrons
The bacterial mobilome consists of accessory genetic elements that move intracellularly or intercellularly. Plasmids provide autonomous replication, transposable elements (IS elements and composite transposons) enable genomic rearrangements, and integrons capture and express exogenous gene cassettes via site-specific recombination.
1. Plasmid Classification & Incompatibility
Plasmids are extrachromosomal, covalently closed circular (ccc) dsDNA molecules replicating autonomously via origins of vegetative replication (oriV). Plasmids belong to the same Incompatibility (Inc) group if they share identical replication initiation or partitioning (Par) mechanisms; when co-introduced into the same host cell, competition for shared regulatory proteins causes stochastic loss of one plasmid line during cell division.
2. Transposable Elements & Jumping Genes
Insertion sequences (IS elements) are the simplest mobile elements, encoding only the transposase flanked by inverted terminal repeats (ITRs). Composite transposons (e.g., Tn10, Tn5) contain two flanking IS elements that mobilize intervening genes encoding antibiotic resistance. Transposition proceeds via conservative (cut-and-paste) or replicative (copy-and-paste) mechanisms, creating direct target site duplications upon integration.
3. Integrons & Multi-Drug Resistance (MDR)
Integrons are natural gene capture systems featuring an integrase gene (intI), an attachment site (attI), and a strong promoter (Pc). Through site-specific recombination between circular non-promoter gene cassettes and attI, integrons stack multiple antibiotic resistance cassettes in tandem, creating multidrug-resistant superbug phenotypes.