4.2 Restriction Endonucleases, Ligases & Nucleases

4.2 Restriction Endonucleases, Ligases & Nucleases

Quick Summary: Restriction endonucleases, ligases, and nucleases are the primary tools used for manipulating and preparing nucleic acids in molecular pathology. Type II restriction endonucleases cleave specific palindromic DNA recognition sites, yielding blunt or sticky overhangs essential for Restriction Fragment Length Polymorphism (RFLP) analysis and recombinant subcloning. DNA ligases repair phosphodiester nicks through ATP- or $\text{NAD}^+$-dependent mechanisms, while diagnostic nucleases (DNase I, RNases, Exonuclease I, and alkaline phosphatases) maintain sample integrity, eliminate off-target nucleic acids, and prepare amplicons for high-throughput sequencing.

---

1. Restriction Endonucleases: Types & Enzymatic Mechanisms

Restriction endonucleases (restriction enzymes) are bacterial defense enzymes that recognize and cleave foreign bacteriophage DNA. Bacteria protect their own chromosomal DNA from self-digestion by expressing cognate DNA methyltransferases that modify specific adenine or cytosine bases within the recognition sequence, establishing a Restriction-Modification (R-M) system.

                           ┌─────────────────────────────────────────┐
                           │      Restriction Endonuclease Types     │
                           └────────────────────┬────────────────────┘
         ┌────────────────────────┬─────────────┴────────────┬────────────────────────┐
         ▼                        ▼                          ▼                        ▼
┌─────────────────┐      ┌─────────────────┐        ┌─────────────────┐      ┌─────────────────┐
│     Type I      │      │     Type II     │        │    Type IIS     │      │     Type IV     │
├─────────────────┤      ├─────────────────┤        ├─────────────────┤      ├─────────────────┤
│ • Multi-subunit │      │ • Homodimers    │        │ • Asymmetric    │        │ • Cleaves only  │
│ • Needs ATP,SAM │      │ • Needs Mg²⁺    │        │   recognition   │        │   methylated/   │
│ • Cuts >1000 bp │      │ • Cuts WITHIN or│        │ • Cuts outside  │        │   modified DNA  │
│   from site     │      │   adjacent site │        │   site (shifted)│        │ • McrBC, MspJI  │
│ • No diagnostic │      │ • RFLP standard │        │ • Golden Gate   │        │ • Epigenetics   │
│   utility       │      │   EcoRI, HindIII│        │   FokI, BsaI    │        │   profiling     │
└─────────────────┘      └─────────────────┘        └─────────────────┘      └─────────────────┘

Classification of Restriction Systems

• Type I Enzymes: Multi-subunit pentameric complexes possessing both restriction endonuclease and methyltransferase activities. They require ATP, S-adenosylmethionine (SAM), and $\text{Mg}^{2+}$ as cofactors. After recognizing a bipartite asymmetric sequence (e.g., 5'-AAC[N6]GTGC-3'), the enzyme translocates DNA and cleaves randomly at distant sites $>1,000,\text{bp}$ away from the recognition motif. They have no utility in diagnostic mapping due to variable cleavage sites.
• Type II Enzymes: The cornerstone of clinical diagnostics and molecular cloning. Typically homodimers that require only $\text{Mg}^{2+}$ as a catalytic cofactor (no ATP or SAM). They recognize symmetric palindromic sequences (4 to 8 bp long) possessing two-fold rotational dyad symmetry and cleave precisely at designated phosphodiester bonds within or immediately adjacent to the recognition sequence.
• Type IIS Enzymes: Subtype of Type II (e.g., FokI, BsaI, BsmBI). They recognize asymmetric, non-palindromic sequences and cleave double-stranded DNA at a precise shifted distance outside the recognition site (e.g., FokI recognizes 5'-GGATG-3' and cleaves 9 nt downstream on the top strand and 13 nt downstream on the bottom strand: GGATG(N)9/(N)13). Type IIS enzymes are essential for Golden Gate modular assembly, TALEN genome editing architectures, and directional NGS library adapter ligations.
• Type III Enzymes: Large heterooligomeric complexes requiring ATP and $\text{Mg}^{2+}$ (stimulated by SAM). Recognize two inverted non-palindromic sequences and cleave 25–27 bp downstream of the recognition site.
• Type IV Enzymes: Cleave only modified (methylated, hydroxymethylated, or glucosyl-hydroxymethylated) DNA while sparing unmodified DNA (e.g., McrBC, MspJI). Used extensively in epigenetic profiling to selectively digest methylated genomic regions.

---

2. Cleavage Geometries & Terminal Nomenclature

Type II restriction endonucleases cleave the phosphodiester backbone to leave distinct structural termini with high-yield diagnostic implications:

5' Overhang (Cohesive / Sticky)       3' Overhang (Cohesive / Sticky)              Blunt End
       EcoRI (G^AATTC)                       PstI (CTGCA^G)                     SmaI (CCC^GGG)

  5'-G         AATTC-3'                5'-CTGCA         G-3'                5'-CCC     GGG-3'
  3'-CTTAA         G-5'                3'-G         ACGTC-5'                3'-GGG     CCC-5'
     ▲             ▲                                ▲     ▲                     ▲       ▲
     └─ 4-nt 5' Overhang                            └─ 4-nt 3' Overhang         └─ No Overhang ─┘

Isoschizomers, Neoschizomers & Isocaudomers

• Isoschizomers: Restriction enzymes isolated from different bacterial species that recognize the identical nucleotide sequence and cleave at the exact same position.
  • Example: SphI (5'-CGTAC^G-3') and BbuI (5'-CGTAC^G-3').
• Neoschizomers: Restriction enzymes that recognize the identical nucleotide sequence but cleave at different positions within that sequence, generating different terminal geometries.
  • Example: SmaI (5'-CCC^GGG-3', yields blunt ends) vs. XmaI (5'-C^CCGGG-3', yields 4-nt 5' overhangs).
• Isocaudomers: Restriction enzymes that recognize different nucleotide sequences but generate identical, compatible cohesive overhangs.
  • Example: BamHI (5'-G^GATCC-3') and BglII (5'-A^GATCT-3'). Both leave a 5'-GATC-3' sticky overhang. When ligated together, the resulting hybrid junction (5'-AGATCC-3') can no longer be cleaved by either BamHI or BglII, effectively destroying both restriction sites.

---

3. Star Activity & Methylation Sensitivity

Star Activity ($st$)

Under non-optimal physicochemical conditions, restriction endonucleases undergo star activity—a loss or relaxation of cleavage specificity resulting in sequence cleavage at non-canonical, degenerate recognition sites (e.g., EcoRI relaxing from 5'-GAATTC-3' to 5'-NAATTC-3' or 5'-GAANTC-3'). Star activity generates spurious, unexpected bands on diagnostic gel electrophoresis that mimic pathogenic structural rearrangements or contamination.

Primary Inducers of Star Activity:

1. High Glycerol Concentration: Storage buffers contain 50% glycerol. If enzyme volume exceeds 5% to 10% of total reaction volume, glycerol induces star activity.
2. High Enzyme-to-DNA Ratio: Overdigestion ($>100,\text{units/}\mu\text{g DNA}$) over prolonged incubation periods.
3. Suboptimal Ionic Strength / Low Salt: Low ionic strength weakens electrostatic specificity between enzyme residues and target base pairs.
4. Elevated pH ($>8.0$): Alkaline buffers alter active-site protonation states.
5. Organic Solvents: Presence of residual ethanol, DMSO, or ethylene glycol carried over from extraction protocols.
6. Substitution of Divalent Cations: Replacing $\text{Mg}^{2+}$ with non-cognate divalent cations ($\text{Mn}^{2+}, \text{Co}^{2+}, \text{Zn}^{2+}$).

Methylation Sensitivity in Clinical Diagnostics

Bacterial and eukaryotic methylases add methyl groups to specific bases, altering restriction enzyme binding:

Unmethylated 5'-CCGG-3'             Methylated 5'-C[5mC]GG-3' (CpG locus)

  HpaII ──▶ [CLEAVES DNA]             HpaII ──▶ [BLOCKED / NO CLEAVAGE]
  MspI  ──▶ [CLEAVES DNA]             MspI  ──▶ [CLEAVES DNA]

• HpaII vs. MspI (Epigenetic & Fragile X Diagnostics): Both enzymes recognize 5'-CCGG-3'.
  • HpaII is methylation-sensitive: it is completely blocked when the internal cytosine is methylated ($5^{\prime}\text{-C}[5\text{mC}]\text{GG-}3^{\prime}$).
  • MspI is methylation-insensitive: it cleaves $5^{\prime}\text{-CCGG-}3^{\prime}$ regardless of internal cytosine methylation.
  • Clinical Application: Southern blot analysis of the FMR1 (Fragile X) promoter and X-chromosome inactivation skewing assays use EcoRI (structural flank) + HpaII (methylation probe) double-digestion to determine whether expanded CGG triplet repeats are transcriptionally silenced by hypermethylation.
• DpnI (Site-Directed Mutagenesis): Specifically requires adenine methylation at $\text{N}^6$ ($5^{\prime}\text{-G}[6\text{mA}]\text{TC-}3^{\prime}$). In PCR-based site-directed mutagenesis, DpnI is added post-amplification to selectively digest the methylated parental plasmid template isolated from dam+ E. coli, sparing the unmethylated newly synthesized mutant plasmid strands.

---

4. DNA & RNA Ligases: Reaction Chemistry & Cofactors

DNA ligases repair single-stranded nicks and seal double-stranded DNA breaks by catalyzing covalent $3^{\prime} \rightarrow 5^{\prime}$ phosphodiester bond formation between adjacent 3'-hydroxyl (3'-OH) and 5'-monophosphate (5'-P) termini.

Step 1: Enzyme Adenylation
        Ligase + ATP (or NAD⁺) ──▶ Ligase-AMP + PPi (or NMN)

Step 2: AMP Transfer to DNA 5'-Phosphate
        Ligase-AMP + 5'-P-DNA ──▶ App-DNA (Adenylated 5' Intermediate) + Ligase

Step 3: Phosphodiester Bond Formation & AMP Release
        DNA-3'-OH + App-DNA ──▶ DNA-Phosphodiester-DNA + AMP

Three-Step Enzymatic Ligation Mechanism

1. Enzyme Activation (Adenylation): The ligase reacts with a nucleotide cofactor (ATP or $\text{NAD}^+$) to form a covalent enzyme-lysyl-AMP intermediate, releasing pyrophosphate ($\text{PP}_i$) or nicotinamide mononucleotide (NMN).
2. AMP Transfer: The AMP moiety is transferred from the enzyme to the 5'-monophosphate group of the donor DNA strand, forming an activated pyrophosphate linkage (AppDNA).
3. Phosphodiester Attack: The 3'-OH of the acceptor strand executes a nucleophilic attack on the 5'-adenylated intermediate, generating a contiguous phosphodiester bond and releasing free AMP.

Board Exam Trap: Cofactor selection is a classic ASCP MB question! Remember: Bacterial ligases (E. coli, Taq ligase) require $\mathbf{NAD^+}$, whereas Bacteriophage (T4, T7) and Eukaryotic ligases require $\mathbf{ATP}$.

---

5. Diagnostic Nucleases & Enzymatic Pre-Analytical Cleanup

Nucleases hydrolyze phosphodiester bonds and are deployed to purify samples, eliminate unwanted nucleic acids, and prepare sequencing templates:

                      ┌───────────────────────────────────────────┐
                      │     Diagnostic Nucleases & Phosphatases   │
                      └─────────────────────┬─────────────────────┘
         ┌──────────────────────────────────┼──────────────────────────────────┐
         ▼                                  ▼                                  ▼
┌─────────────────────────┐    ┌─────────────────────────┐    ┌─────────────────────────┐
│         DNases          │    │         RNases          │    │ Exonucleases / Phosph.  │
├─────────────────────────┤    ├─────────────────────────┤    ├─────────────────────────┤
│ • DNase I: Cleaves ds/  │    │ • RNase A: ssRNA @ pyrim│    │ • Exo I: 3'➔5' ssDNA    │
│   ssDNA non-specifically│    │ • RNase H: Cleaves RNA  │    │ • Exo III: 3'➔5' dsDNA  │
│ • Cleans RNA preps      │    │   in RNA:DNA hybrids    │    │ • Lambda Exo: 5'➔3' phos│
│ • Inactivated by EDTA/  │    │ • RNase III: Cleaves    │    │ • SAP / CIP: Cleaves 5'P│
│   heat (65°C + EGTA)    │    │   dsRNA (Dicer)         │    │ • ExoSAP: Post-PCR clean│
└─────────────────────────┘    └─────────────────────────┘    └─────────────────────────┘

1. Deoxyribonucleases (DNases)

• Bovine Pancreatic DNase I: Endonuclease that hydrolyzes double-stranded and single-stranded DNA to produce 5'-phosphorylated di-, tri-, and oligonucleotides.
  • Clinical Application: Digestion of trace genomic DNA in total RNA preparations prior to RT-qPCR or RNA-Seq. (Must be inactivated with EDTA/heat or removed via silica spin column before reverse transcription).

2. Ribonucleases (RNases)

• Ribonuclease A (RNase A): Endoribonuclease that specifically cleaves single-stranded RNA after pyrimidine (cytosine and uracil) residues ($3^{\prime}\text{-pyrimidine-P}$). Extremely resilient; boiling does not denature RNase A. Used to remove RNA contamination from plasmid DNA preparations.
• Ribonuclease H (RNase H): Specifically hydrolyzes the phosphodiester bonds of RNA when hybridized to a complementary DNA strand (RNA:DNA duplex), leaving 5'-phosphate and 3'-hydroxyl termini. Does not cleave single-stranded RNA, double-stranded RNA, or double-stranded DNA. Essential for: (1) Second-strand cDNA synthesis by creating RNA primer fragments; (2) Transcription-Mediated Amplification (TMA) and NASBA isothermal diagnostics; (3) Degrading target mRNA after antisense oligonucleotide hybridization.

3. Exonucleases & Phosphatases in Sequencing Cleanup

• Exonuclease I (Exo I): Catalyzes the stepwise removal of mononucleotides from single-stranded DNA strictly in the 3' $\rightarrow$ 5' direction. It does not digest double-stranded DNA or RNA.

• Alkaline Phosphatases (Shrimp Alkaline Phosphatase [SAP], Calf Intestinal Phosphatase [CIP]): Hydrolyze 5'-phosphate groups from DNA, RNA, and dNTPs, leaving 5'-hydroxyl ends. Dephosphorylating linearized plasmid vectors prevents self-ligation.

• ExoSAP-IT Enzymatic Post-PCR Cleanup:

  • Combines Exo I (which digests unconsumed single-stranded PCR primers) and SAP (which dephosphorylates remaining dNTPs into deoxynucleosides and inorganic phosphate).
  • Incubation at 37°C for 15 minutes cleans the PCR product.
  • Subsequent heating to 80°C for 15 minutes irreversibly denatures both enzymes.
  • The cleaned PCR amplicon can be used directly in Sanger cycle sequencing without column purification.

• Lambda ($\lambda$) Exonuclease: A 5' $\rightarrow$ 3' exonuclease that processively digests double-stranded DNA strictly from 5'-phosphorylated termini. Unphosphorylated (5'-OH) strands or single-stranded DNA are highly resistant. Used to selectively destroy one strand of a PCR amplicon (generated with one 5'-phosphorylated primer and one unphosphorylated primer) to produce pure single-stranded DNA for pyrosequencing or SELEX aptamer selection.