Source: Comprehensive Guide to DNA Recombination, Gene Expression, and Regulation (UCF)
Tags: site-specific recombination, SSR, tyrosine recombinase, serine recombinase, Cre-lox, loxP, Hin recombinase, phase variation, Xer recombinase, dif site, transposon, transposable element, transposase, cut-and-paste, replicative transposition, retrotransposon, LINE, SINE, LTR, V(D)J recombination, RAG1, RAG2, RSS, Barbara McClintock
Difficulty: Intermediate Prerequisites: DNA Recombination and Repair notes (Part 1). You should understand Holliday junctions, crossover vs. non-crossover, and the general concept of recombinases before starting here.
Beyond homologous recombination, cells use two other broad classes of genetic rearrangement: site-specific recombination (SSR) and transposition. SSR operates at defined DNA sequences and is used for phage integration, gene regulation by inversion, and chromosome maintenance. Transposition involves mobile genetic elements that move within and between genomes, reshaping genome architecture and driving evolution. V(D)J recombination, which generates antibody and T-cell receptor diversity, borrows elements from both. Together, these mechanisms explain a remarkable amount of genome plasticity.
Site-specific recombination uses dedicated recombinases to cut and rejoin DNA at specific short sequences, producing insertions, deletions, or inversions. Transposable elements are mobile DNA segments that move by cut-and-paste, replicative, or retrotransposition mechanisms, making up nearly half the human genome. V(D)J recombination applies similar logic to assemble diverse immune receptor genes.
Site-specific recombination (SSR)
Recombination that occurs at short, defined DNA sequences (recombination sites) catalysed by dedicated recombinases. Does not require extensive homology. Think of it as a precise cut-and-swap at pre-programmed addresses in the genome.
Recombination recognition sequence (RRS)
Short (~20 nt), asymmetric DNA sequences flanked by binding sites for recombinases. The sites where SSR enzymes cut and rejoin.
Tyrosine recombinase
A class of SSR enzyme that cuts and rejoins one strand pair at a time, forming a Holliday junction intermediate. Named for the catalytic tyrosine residue. Examples: Cre, lambda integrase, FLP.
Serine recombinase
A class of SSR enzyme that cleaves all four strands simultaneously and induces a 180-degree rotation for strand exchange. No Holliday junction intermediate is formed. Examples: Hin, phiC31 integrase.
Cre-lox system
A site-specific recombination tool derived from phage P1. The Cre recombinase (tyrosine family) recognises loxP sites. Widely used for conditional gene knockouts in mice.
loxP site
A 34 bp DNA sequence recognised by Cre recombinase. When two loxP sites flank a gene segment in the same orientation, Cre excises the intervening DNA.
Hin recombinase
A serine recombinase from Salmonella that mediates reversible inversion of a ~1,000 bp DNA segment, controlling expression of alternate flagellin genes (FliC and FljB) for immune evasion (phase variation).
Xer recombinase (XerCD)
A tyrosine recombinase system in bacteria that resolves chromosome dimers into monomers at dif sites before cell division. Requires the auxiliary protein FtsK for directionality.
Transposable element (transposon)
A DNA segment that can move from one genomic location to another. Discovered by Barbara McClintock in maize. Movement is catalysed by transposases.
Transposase
The enzyme that catalyses the excision and/or insertion of a transposon.
Cut-and-paste transposition
A transposition mechanism where the element is excised from its original site and inserted elsewhere, leaving a DSB at the donor site.
Replicative transposition
A transposition mechanism where the element is duplicated during the move; a copy remains at the original site while a new copy appears at the target. Produces a cointegrate intermediate.
Retrotransposon
A transposable element that moves via an RNA intermediate: the element is transcribed into RNA, reverse-transcribed into DNA, and inserted at a new site.
Long terminal repeat (LTR)
Repeated sequences flanking LTR retrotransposons. LTR retrotransposons structurally resemble retroviruses and encode reverse transcriptase and integrase.
LINE (Long Interspersed Nuclear Element)
An autonomous non-LTR retrotransposon (~6 kb) that encodes its own reverse transcriptase and endonuclease. LINE-1 is the most abundant in humans.
SINE (Short Interspersed Nuclear Element)
A non-autonomous retrotransposon (~300 bp) that depends on LINE-encoded enzymes for its mobilisation. Alu elements are the best-known human SINEs.
Target site duplication (TSD)
A short (2-10 bp) duplication of host DNA flanking a newly inserted transposon, generated during the insertion and repair process. A hallmark of transposition.
V(D)J recombination
A form of site-specific recombination in developing lymphocytes that assembles variable (V), diversity (D), and joining (J) gene segments to produce diverse antigen receptors (immunoglobulins and T-cell receptors).
Recombination signal sequence (RSS)
Conserved sequences flanking V, D, and J gene segments. Composed of a heptamer, a spacer (12 or 23 bp), and a nonamer. Recognised by RAG proteins.
RAG1/RAG2
Recombination-activating gene proteins that recognise RSSs, cleave DNA, and form hairpin intermediates during V(D)J recombination. Expression is restricted to developing lymphocytes.
Site-specific recombination (SSR): precise exchange at defined sequences; does not require extended homology
Transpositional recombination (transposition): movement of mobile elements via DNA or RNA intermediates; target sites are less specific
Occurs at short recombination sites with recognition sequences
Catalysed by recombinases of two families: tyrosine or serine
Three possible outcomes depending on the orientation and location of sites:
Insertion: integration of a circular DNA into a chromosome (e.g., phage lambda integration)
Excision/deletion: removal of the DNA segment between two directly oriented sites
Inversion: flipping of the DNA segment between two inversely oriented sites
Essential for phage life cycles, immune diversity, chromosome segregation, and antigenic variation
Tyrosine recombinases:
Cut and rejoin one strand pair at a time
Form a Holliday junction intermediate
Resolution of the junction completes the reaction
Examples: Cre, lambda Int, FLP, XerCD
Serine recombinases:
Cleave all four strands simultaneously
Subunits rotate 180 degrees relative to each other for strand exchange
No Holliday junction intermediate
Often used in genetic engineering (e.g., phiC31 integrase)
Examples: Hin, Gin, phiC31
Derived from bacteriophage P1
Cre is a tyrosine recombinase; loxP is its 34 bp recognition site
Used for conditional gene knockout in mice:
Flank the target gene's critical exons with loxP sites ("floxed" allele)
Cross the floxed mouse with a Cre-expressing transgenic line (Cre driven by a tissue-specific or inducible promoter)
Cre excises the floxed segment only in cells where it is expressed, producing tissue-specific or time-specific gene inactivation
Enables precise, reversible genetic modifications that constitutive knockouts cannot achieve
Hin (serine recombinase) mediates reversible inversion of a ~1,000 bp DNA segment
Controls expression of alternate flagellin genes: FliC and FljB
Inversion switches the promoter orientation, toggling which flagellin is transcribed
Requires the Fis protein and negative DNA supercoiling
This phase variation helps Salmonella evade the host immune system
Homologous recombination between sister chromosomes can produce chromosome dimers in bacteria
Dimers must be resolved into monomers before cell division
XerCD recombinase acts at dif sites (located at the replication terminus)
The auxiliary protein FtsK ensures resolution occurs in the correct orientation
Failure to resolve dimers leads to unequal segregation and cell death
Discovered transposons ("jumping genes") in maize in the 1940s-50s
Transposons can cause mutations, genome rearrangements, and changes in gene regulation
Initially controversial; now understood to be a major force in genome evolution
Transposable elements make up roughly 45% of the human genome
Cut-and-paste:
Transposase cleaves both ends of the element, forming hairpin intermediates
The excised element inserts into a new target site
Repair of the staggered cuts at the target produces target site duplications (2-10 bp)
Leaves a DSB at the donor site (repaired by host machinery)
Replicative:
Transposase nicks the element ends, exposing 3' hydroxyls
Strand transfer joins element ends to the target, creating a cointegrate
DNA synthesis duplicates the transposon: one copy at the old site, one at the new
Retrotransposition:
The element is transcribed into RNA by host RNA polymerase
Reverse transcriptase converts the RNA back into DNA
The new DNA copy is inserted into the genome, often with LTRs
Bacterial:
Insertion sequences (IS): the simplest; contain only the transposase gene flanked by inverted repeats
Composite transposons: IS elements flanking additional genes (e.g., antibiotic resistance)
Complex transposons: larger elements with regulatory genes
Eukaryotic:
DNA transposons: similar to bacterial elements; move by cut-and-paste
Retrotransposons: move via RNA intermediate; dominate in humans (~46% of genome)
LTR retrotransposons: resemble retroviruses; encode reverse transcriptase and integrase; flanked by long terminal repeats
Non-LTR retrotransposons:
LINEs (e.g., LINE-1): autonomous (~6 kb); encode their own reverse transcriptase and endonuclease
SINEs (e.g., Alu): non-autonomous (~300 bp); rely on LINE-encoded machinery for mobilisation
Together, retrotransposons constitute a large fraction of the human genome and have significantly influenced genome size and evolution
Generates the enormous diversity of antibodies and T-cell receptors from a limited set of gene segments
V (variable), D (diversity), and J (joining) segments are arranged in clusters, each flanked by recombination signal sequences (RSS)
The 12/23 rule: recombination occurs only between an RSS with a 12 bp spacer and one with a 23 bp spacer, enforcing ordered assembly
Catalysed by RAG1/RAG2 proteins:
Recognise and bind RSS
Introduce DNA breaks, forming hairpin structures at the coding ends
Coding ends are joined by NHEJ machinery, often with added nucleotide diversity (junctional diversity)
The combinatorial joining of different V, D, and J segments, plus junctional diversity, produces an almost limitless repertoire of antigen receptors
RAG activity is tightly regulated to prevent dangerous off-target rearrangements
RAG1/RAG2 expression is restricted to developing lymphocytes (lymphocyte precursors in bone marrow and thymus)
Misregulation can lead to activation of oncogenes or chromosomal translocations associated with leukaemia and lymphoma
RAG proteins are thought to have evolved from an ancient transposase, linking V(D)J recombination to transposition
The Cre-lox system is one of the most widely used tools in mouse genetics, enabling researchers to knock out genes in specific tissues or at specific developmental stages. Understanding transposable elements has implications for gene therapy (some viral vectors use LTR-based integration) and for understanding genomic diseases caused by LINE or Alu insertions. V(D)J recombination is fundamental to immunology and explains how a finite genome encodes the capacity to recognise virtually any pathogen.
Students often confuse tyrosine and serine recombinase mechanisms. The key distinction: tyrosine recombinases form a Holliday junction intermediate (one strand pair at a time); serine recombinases cut all four strands and rotate, with no Holliday junction.
Cre-lox is sometimes described as "deleting genes everywhere." In conditional knockouts, Cre expression is tissue-specific or inducible, so deletion occurs only in defined cell populations.
Students sometimes assume all transposons use a cut-and-paste mechanism. Retrotransposons (which dominate the human genome) use a copy-and-paste mechanism via an RNA intermediate.
V(D)J recombination is occasionally confused with class switch recombination or somatic hypermutation. V(D)J assembles the initial receptor; class switching and hypermutation refine it later in B cells.
⚠️ Know the difference between tyrosine and serine recombinase mechanisms (Holliday junction vs. rotation).
⚠️ Understand the three outcomes of SSR (insertion, deletion/excision, inversion) and how repeat orientation determines the outcome.
⚠️ Be able to explain the Cre-lox conditional knockout workflow.
⚠️ Distinguish cut-and-paste, replicative, and retrotransposition mechanisms.
⚠️ The 12/23 rule in V(D)J recombination is a commonly tested concept.
⚠️ Know that RAG proteins are restricted to lymphocyte precursors and why that matters.
True or false: Serine recombinases form Holliday junction intermediates.
Fill in the blank: The Cre recombinase recognises __________ sites.
True or false: SINEs encode their own reverse transcriptase.
Fill in the blank: V(D)J recombination is catalysed by the __________ proteins.
True or false: Target site duplications are produced during transposon insertion.
Answers: 1. False (no Holliday junction; they rotate). 2. loxP. 3. False (SINEs are non-autonomous and rely on LINE machinery). 4. RAG1/RAG2. 5. True.
Q: How do tyrosine and serine recombinases differ mechanistically?
A: Tyrosine recombinases cleave and rejoin one strand pair at a time, forming a Holliday junction intermediate that is then resolved. Serine recombinases cleave all four strands simultaneously, rotate subunits 180 degrees, and re-ligate, producing the recombinant product in a single concerted step without a Holliday junction.
Q: Explain how the orientation of recombination sites determines the outcome of SSR.
A: If two sites are in the same orientation (direct repeats), recombination excises/deletes the intervening segment as a circular molecule. If the sites are in inverted orientation, recombination inverts the intervening segment. If the sites are on separate molecules (e.g., a chromosome and a circular phage genome), recombination integrates (inserts) one molecule into the other.
Q: Describe the Cre-lox conditional gene knockout strategy.
A: The target gene's critical exon(s) are flanked by loxP sites in the same orientation (a "floxed" allele) using homologous recombination in embryonic stem cells. Mice carrying the floxed allele are crossed with transgenic mice expressing Cre recombinase under a tissue-specific or inducible promoter. In cells where Cre is active, it recognises the loxP sites and excises the intervening DNA, inactivating the gene. Cells that do not express Cre retain normal gene function.
Q: What is the 12/23 rule in V(D)J recombination, and what is its purpose?
A: The 12/23 rule states that RAG-mediated recombination occurs only between a gene segment flanked by an RSS with a 12 bp spacer and one flanked by an RSS with a 23 bp spacer. This prevents joining of segments within the same category (e.g., two V segments) and ensures the correct order of assembly (V to D, D to J).
Q: Why do retrotransposons dominate the human genome rather than DNA transposons?
A: Retrotransposons use a copy-and-paste mechanism, meaning the original element remains in place while a new copy is inserted elsewhere. Over evolutionary time this leads to net expansion. DNA transposons use cut-and-paste, which moves the element without increasing copy number, so their numbers remain relatively stable.
This material connects back to HR and NHEJ (Part 1), since V(D)J recombination relies on NHEJ for the joining step, and transposon excision leaves DSBs repaired by the same pathways. It also connects forward to gene regulation (Parts 3 and 4), since site-specific inversions (Hin/phase variation) and transposon insertions are mechanisms of gene regulation. The Cre-lox system is also a bridge to genetic engineering and CRISPR-based approaches.
site-specific recombination, SSR, tyrosine recombinase, serine recombinase, Cre recombinase, loxP, conditional knockout, floxed, Hin recombinase, phase variation, flagellin, FliC, FljB, Fis, XerCD, dif site, FtsK, chromosome dimer resolution, transposon, transposable element, jumping gene, Barbara McClintock, transposase, cut-and-paste transposition, replicative transposition, cointegrate, retrotransposon, retrotransposition, reverse transcriptase, LTR, long terminal repeat, LINE, LINE-1, SINE, Alu element, target site duplication, insertion sequence, IS element, composite transposon, V(D)J recombination, RAG1, RAG2, RSS, recombination signal sequence, 12/23 rule, junctional diversity, immunoglobulin, T-cell receptor, immune diversity