Recombinant Saccharomyces cerevisiae DNA topoisomerase 1 (TOP1), partial

Definition and Basic Properties

Recombinant Saccharomyces cerevisiae DNA topoisomerase 1 (TOP1), partial refers to a truncated form of the S. cerevisiae Top1 enzyme produced via recombinant DNA technology. This protein retains functional domains necessary for its DNA relaxation activity but lacks full-length sequence completeness . Key characteristics include:

Biological Role of Top1 in S. cerevisiae

Top1 resolves DNA torsional stress by creating transient single-strand breaks, enabling controlled rotation of DNA strands . Its activity is implicated in:

• Transcription-Associated Mutagenesis (TAM): High transcription levels increase Top1-mediated short deletions (−2/−3 nt) at repetitive sequences, linked to trapped Top1-DNA covalent complexes .

• Ribonucleotide Excision Repair (RER): Top1 processes unrepaired ribonucleotides embedded in DNA, leading to error-prone deletions or error-free repair via coordinated pathways .

• Replication Conflicts: Collisions between Top1-DNA intermediates and replication forks generate DNA double-strand breaks (DSBs), necessitating repair by Rad52-dependent homologous recombination .

Catalytic Activity and Substrate Specificity

• RNA-DNA Hybrid Processing: Top1 incises DNA at ribonucleotide sites, forming 2′,3′-cyclic phosphate termini. This activity is sequence-dependent, favoring repetitive motifs like (GA)₄ or (TC)₄ .

• Error-Prone vs. Error-Free Repair:

  • Error-prone: Top1 cleavage at ribonucleotides triggers −2/−3 nt deletions via template misalignment during repair synthesis .

  • Error-free: Tdp1 and Tpp1 resolve Top1-DNA adducts, restoring 3′-OH termini for accurate ligation .

Interaction with DNA Repair Proteins

• Rad1-Rad10 and Mus81-Mms4: These endonucleases process Top1-induced DNA gaps, with their absence reducing short deletion rates by ~50% .

• Poly(ADP-ribose) Polymerase 1 (PARP1): Modulates Top1-DNA adduct stability via ADP-ribosylation, influencing repair outcomes .

Applications in Research

Recombinant Top1 is utilized to:

1. Map Cleavage Hotspots: Identify Top1-preferred sequences (e.g., 5′-(A/T)(G/C)(A/T)-3′) using oligonucleotide substrates .

2. Study Ribonucleotide-Induced Mutagenesis: Reconstitute Top1-dependent deletion formation in vitro to model genomic instability .

3. Screen Anticancer Drug Targets: Evaluate compounds that stabilize Top1-DNA complexes (e.g., camptothecin analogs) .

Technical Considerations

• Enrichment of Covalent Complexes: Chromatin immunoprecipitation (ChIP) assays reveal Top1 accumulation at highly transcribed loci, correlating with mutation hotspots .

• Activity Assays: Gel electrophoresis and radiolabeled DNA substrates quantify Top1 cleavage/religation kinetics .

Limitations and Future Directions

• Partial Protein Constraints: The truncated recombinant form may lack regulatory domains affecting in vivo behavior.

• Ribonucleotide Context: Sequence position and flanking bases strongly influence Top1 cleavage efficiency, necessitating substrate-specific analyses .