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Mismatch excision and possible polarity effects result in preferred deoxyribonucleic acid strand of integration in
Abstract:
Heteroduplex deoxyribonucleic acid molecules having a drug resistance marker on one strand and its wild-type allele on the other have been used as donors in pneumococcal transformation. Opposite strands are not equally effective in producing transformants, and this strand bias is not the same, either in direction or magnitude, for various different genetic markers. Selective excision of mismatched base pairs is probably responsible for the large differences in strand efficiency seen with discriminating (hex+) strains, for when the recipient is nondiscriminating (hex-), and therefore presumably lacking an excision enzyme system, strand bias is drastically reduced or altered. The evidence also indicates that excision occurs after integration, as it is provoked by specific donor-recipient mismatch and not by the same mismatch when introduced within donor heteroduplex molecules. Excision can extend to include a neighboring linked marker which would otherwise not be excised, thus altering its intrinsic strand bias as well as its efficiency in transformation. There is a small bias in relative strand efficiency for some markers, not caused by mismatch excision, which perhaps is due to polarity in the integration process itself.
Insights
Pneumococcal transformation shows strand bias in DNA repair. Mismatched base excision after integration explains most strand bias, affecting linked markers and transformation efficiency.
Area of Science:
- Molecular Biology
- Genetics
- Microbiology
Background:
- Bacterial transformation is a key mechanism for genetic exchange.
- Heteroduplex DNA molecules, with mismatched strands, are used as donors in transformation.
- Strand bias, where one DNA strand is more effective than the other, is observed in transformation.
Purpose of the Study:
- To investigate the mechanisms underlying strand bias in pneumococcal transformation.
- To determine the role of mismatch excision in strand bias.
- To understand how excision affects linked genetic markers during transformation.
Main Methods:
- Utilizing heteroduplex deoxyribonucleic acid (DNA) molecules with drug resistance markers and wild-type alleles as donors.
- Comparing transformation efficiencies between different genetic markers and recipient strains (hex+ vs. hex-).
- Analyzing the effect of selective excision of mismatched base pairs on strand bias.
Main Results:
- Opposite DNA strands exhibit unequal effectiveness in producing transformants, with varying strand bias across different genetic markers.
- Selective excision of mismatched base pairs in discriminating (hex+) strains is a primary cause of significant strand bias.
- Strand bias is drastically reduced or altered in nondiscriminating (hex-) strains, suggesting a role for excision enzymes.
- Excision occurs post-integration and can extend to linked markers, altering their transformation efficiency and intrinsic strand bias.
- A minor strand bias, unrelated to mismatch excision, may result from integration process polarity.
Conclusions:
- Selective mismatch excision after DNA integration is the main driver of strand bias in pneumococcal transformation.
- The hex+ system in Streptococcus pneumoniae plays a crucial role in mediating strand bias through post-integrational mismatch repair.
- Understanding strand bias and excision mechanisms is vital for comprehending DNA repair and genetic exchange in bacteria.