Class 1 Integron (intI1, sul1) and IS26 Composite Transposons

Class 1 Integrons (intI1–attI1–qacEΔ1–sul1) and Insertion Sequence IS26 constitute the primary two-tier genetic engineering and dissemination platform driving clinical multidrug resistance (MDR) across Gram-negative Enterobacterales (Escherichia coli, Klebsiella pneumoniae, Salmonella enterica, Enterobacter spp.), Pseudomonas aeruginosa, and Acinetobacter baumannii. First characterized by Ruth M. Hall and H. W. Stokes in 1989, integrons are site-specific genetic capture and expression systems that are not autonomously mobile on their own, but instead reside within transposons (Tn402, Tn21), integrative and conjugative elements (ICEs), and conjugative plasmid replicons.

Whereas the Class 1 integron operates as a molecular cassette deck—capturing promoterless circular open reading frames (gene cassettes) at a specific receptor site (attI1) and placing them under the control of a strong outward-facing promoter (P_c) to form a co-transcribed multidrug resistance operon—the 820-bp IS26 element (IS6 family) acts as a high-frequency structural remodeler. Together, intI1 and IS26 assemble complex Multiple Antibiotic Resistance Regions (MARRs) carrying blaNDM-1, blaKPC-2, blaCTX-M-15, aminoglycoside-modifying enzymes, and trimethoprim/sulfonamide resistance cassettes through horizontal gene transfer.

The 5′-Conserved Segment (intI1, attI1, P_c), Variable Array, and Clinical 3′-CS (qacEΔ1–sul1)

A canonical clinical Class 1 integron (descended from an environmental Betaproteobacteria chromosomal ancestor captured into a Tn402/Tn5090-like mercury-resistance transposon backbone) consists of three modular zones:

1. The 5′-Conserved Segment (5′-CS, ~1.36 kb): Encodes intI1 (1,014 bp, encoding the 337-amino-acid tyrosine site-specific recombinase IntI1), transcribed inward from promoter P_int, and the adjacent primary recombination receptor site attI1 (~65 bp, containing a core 7-bp crossover sequence 5′-GTTAGGC-3′ flanked by three IntI1-binding domains). Embedded within the coding sequence of intI1 (or in the attI1 region) is the outward-facing cassette promoter P_c (existing in four main natural allelic variants—P_cW weak, P_cH1 hybrid-1, P_cH2 hybrid-2, and P_cS strong, differing by a 20-fold range in transcriptional strength depending on their −35 and −10 hexamer sequences), which drives transcription of all captured downstream cassettes. Because P_c and P_int overlap or sit face-to-face, mutations that weaken P_c simultaneously derepress intI1 transcription via transcriptional interference, coupling low cassette expression to elevated integrase activity.
2. The Variable Gene Cassette Array (0 to >8 cassettes, ~0.5 to 8 kb): A tandem head-to-tail array of mobile gene cassettes. Each individual cassette (~500–1,500 bp) comprises a single promoterless open reading frame (ORF) immediately followed at its 3′ end by an imperfect palindromic recombination site known as attC (historically called the 59-base element, ranging in length from 57 to 141 bp and bounded by conserved inverse core R″ 5′-RYYYAAC-3′ and core R′ 5′-GTTRRRY-3′ heptamers). Over 130 distinct resistance cassettes have been cataloged in clinical Class 1 integrons, including aminoglycoside adenylyltransferases and acetyltransferases (aadA1, aadA2, aadB, aac(6′)-Ib-cr), dihydrofolate reductases (dfrA1, dfrA12, dfrA17 conferring trimethoprim resistance), Class B1 metallo-β-lactamases (blaVIM-2, blaIMP-1), Class D oxacillinases (blaOXA-1, blaOXA-10), and chloramphenicol efflux/acetyltransferases (catB3, cmlA1).
3. The 3′-Conserved Segment (3′-CS, ~2.0 kb): Formed during the evolutionary assembly of clinical Class 1 integrons when the 3′-end of an ancestral qacE gene cassette (encoding a small multidrug resistance SMR-family efflux pump exporting quaternary ammonium compounds, hospital surface disinfectants, and intercalating dyes) was truncated by insertion of the sul1 gene (encoding a sulfonamide-insensitive dihydropteroate synthase that bypasses sulfamethoxazole inhibition of folate synthesis), followed by orf5 (an acetyltransferase of unknown function) and often an ISCR1 rolling-circle element.

The Folded Single-Stranded Bottom-Strand (attC_bs) Recombination Mechanism

Whereas canonical tyrosine recombinases (such as bacteriophage λ integrase or Cre/Flp) recombine two double-stranded DNA (dsDNA) partner sites (attP × attB), structural and biochemical studies by Didier Mazel, Alfonso Mondragón, and colleagues revealed that IntI1 catalyzes an asymmetric recombination reaction between one double-stranded site (attI1) and one folded single-stranded DNA (ssDNA) hairpin formed by the bottom strand of attC (attC_bs).

During bacterial conjugation (which transfers a single DNA strand into the recipient cell), natural transformation, or lagging-strand DNA replication across the palindromic attC sequence, the single-stranded bottom strand of attC extrusion-folds into an imperfect stem-loop hairpin containing two conserved Extrahelical Bases (EHBs, typically a flipped-out purine G and T) separated by a central unpaired bubble. An IntI1 tetramer specifically recognizes and binds the two extrahelical bases of the folded attC_bs hairpin (IntI1 has virtually zero affinity for canonical base-paired dsDNA attC), pairing it with the double-stranded attI1 receptor site. Two catalytic Tyr312 residues cleave one strand of each partner (5′-G↓TTAGGC-3′) to form 3′-phosphotyrosyl covalent intermediates and resolve the resulting single-strand Holliday junction (aHJ) via a passage of a replication fork.

Because new gene cassettes are preferentially inserted at the attI1 × attC junction immediately downstream of the P_c promoter, the most recently captured resistance gene occupies the first position of the array and receives the highest level of mRNA transcription, while older cassettes pushed toward the 3′-CS experience progressive transcriptional attenuation across the imperfect attC stem-loops. Furthermore, intI1 promoter contains a LexA-binding SOS box: exposure to sub-inhibitory fluoroquinolones (ciprofloxacin), trimethoprim, or β-lactams triggers RecA-mediated autocleavage of the LexA repressor, inducing IntI1 expression by up to 35-fold so the bacterium shuffles its cassette order or excises/captures new cassettes precisely under antibiotic stress.

Replicative Cointegrate Formation vs. Conservative Targeted Translocatable Units (TUs)

Complementing the cassette-level precision of IntI1 is IS26, an 820-bp insertion sequence belonging to the IS6 family bounded by perfect 14-bp terminal inverted repeats (TIRs: 5′-GGCACTGTTGCAAA-3′) and encoding a single 234-amino-acid DDE-motif transposase (Tnp26). Unlike cut-and-paste insertion sequences (IS10, IS50), IS26 lacks a mechanism for simple non-replicative excision of a single IS copy. Instead, Ruth M. Hall, Christopher J. Harmer, and colleagues demonstrated that Tnp26 operates via two distinct pathways:

1. Unimolecular / Bimolecular Replicative Transposition (Cointegrate Formation): Tnp26 cleaves the 3′-OH ends of a single IS26 element and attacks a random target DNA site, generating a Shapiro intermediate whose replication fuses the donor and target molecules into a cointegrate in which the two replicons are joined by two directly oriented copies of IS26 flanked by an 8-bp Target Site Duplication (TSD). Intramolecular replicative transposition within the same plasmid or chromosome either deletes the intervening segment (producing a circular Translocatable Unit, TU, consisting of a single IS26 plus adjacent resistance DNA) or inverts the DNA segment (when the target site is attacked in the opposite orientation).
2. Targeted Conservative Integration (The High-Frequency TU Pathway): When a circular Translocatable Unit (TU) containing one IS26 and a resistance gene (such as blaNDM-1–bleMBL or blaSHV-12) encounters a recipient plasmid or chromosomal island that already contains a pre-existing copy of IS26, Tnp26 catalyzes a conservative, site-specific reaction between the two IS26 copies (Tnp26-mediated targeted integration, which occurs at a 50-fold to 100-fold higher frequency than untargeted replicative transposition and is independent of RecA). This reaction inserts the TU directly adjacent to the resident IS26 without creating a new 8-bp TSD, producing a tandem IS26–cargo–IS26 pseudo-compound transposon (such as Tn4352 carrying aphA1a kanamycin resistance) and building nested mosaic arrays of up to 10–15 interspersed IS26 copies and resistance cassettes.

Tandem Gene Amplification of β-Lactamases Under Antibiotic Pressure

The targeted TU integration mechanism and homologous recombination between direct IS26 repeats endow clinical Gram-negative bacteria with a rapid gene-dosage amplification switch. When a Klebsiella pneumoniae or Escherichia coli isolate carrying an IS26-bounded β-lactamase (blaTEM-1, blaSHV-1, blaOXA-1, blaCTX-M-15, or blaNDM-1) is exposed to piperacillin–tazobactam, ceftazidime–avibactam, or meropenem, IS26-mediated TU rolling/re-insertion or RecA-dependent unequal crossing-over generates tandem head-to-tail arrays of 5 to 50 copies of the IS26–bla unit within a few generations (growth phase copy-number expansion).

This 10-fold to 50-fold increase in β-lactamase gene copy number hyper-produces the enzyme in the periplasm, stoichiometricallly titrating out suicide β-lactamase inhibitors (tazobactam, clavulanate, sulbactam) and overwhelming porin-restricted carbapenem influx (inoculum and gene-dosage heteroresistance), only to collapse back to a low-fitness single copy once antibiotic selective pressure is withdrawn.

Why are intI1 and sul1 used globally as molecular biomarkers of anthropogenic antibiotic pollution?

In pristine natural soils and pre-antibiotic bacterial collections, chromosomal integrons exist in diverse environmental forms lacking qacEΔ1 and sul1. The specific intI1–qacEΔ1–sul1 clinical Class 1 integron structure was selected in human medicine during the mid-20th century following the introduction of quaternary ammonium hospital disinfectants (selecting qacE) and sulfonamide antibiotics (selecting sul1). Because this clinical Class 1 integron is now linked to genes conferring resistance to nearly all major antibiotic classes, heavy metals (mer operon), and biocides—and is shed in high concentrations (10^6 to 10^10 copies per gram) in hospital wastewater, municipal sewage, and agricultural manure—environmental microbiologists use quantitative PCR (qPCR) of intI1 and sul1 normalized to 16S rRNA as the standard proxy for anthropogenic antimicrobial-resistance pollution.

How does a gene cassette get transcribed if it lacks its own promoter?

Individual circular gene cassettes consist almost exclusively of an open reading frame (ORF) and a 3′ attC recombination site, with no upstream promoter (−35/−10 hexamers) and often no canonical Shine–Dalgarno ribosome-binding site. Therefore, a free circular gene cassette is transcriptionally silent. When IntI1 integrates the cassette at the attI1 site of the 5′-CS, the cassette ORF is positioned directly downstream of the outward-facing integron cassette promoter P_c (located within the intI1 coding sequence). Transcription initiated at P_c reads through attI1 and across the entire tandem cassette array as a polycistronic mRNA, while a short conserved open reading frame (ORF-11) overlapping attI1 enhances translation re-initiation at leaderless or Shine–Dalgarno-poor first-position cassettes.

What is the difference between a classical composite transposon (like Tn5 or Tn10) and an IS26 pseudo-compound transposon?

In a classical composite transposon such as Tn5 (flanked by IS50) or Tn10 (flanked by IS10), the transposase moves the entire IS–cargo–IS structure as a single cut-and-paste unit by recognizing the two outer ends of the flanking IS elements. In contrast, an IS26-flanked structure (IS26–cargo–IS26, now termed a pseudo-compound transposon or translocatable unit TU) moves via an excised or replicatively generated circular intermediate containing only a single copy of IS26 plus the resistance cargo; it reconstructs the two-IS26 bracketed structure only after Tnp26 inserts that circular TU alongside another IS26 copy in the target replicon.

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