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Mutants of Shigella sonnei deficient in DNA polymerase I
Abstract:
Mutants of Shigella sonnei (S. sonnei) deficient in DNA polymerase I were isolated after mutagenesis with nitrosoguanidine. The isolation of the mutants was facilitated by the use of a strain harboring plasmid pBR313 which required DNA polymerase I for its muliplication. The mutants isolated could not maintain the plasmid and became sensitive to methyl methanesulfonate (MMS) and to ultraviolet light (UV) irradiation. Assays performed on crude extracts established that the mutants were deficient in an enzyme with DNA polymerase activity. All of these properties are the same as those of E. coli polA. Several MMS-resistant revertants isolated from one of the S. sonnei polA mutants regained 3-120% of the DNA polymerase activity found in the extracts of the wild-type parent strain. Most though not all of the revertants could support the multiplication of plasmid pBR313.
Insights
Researchers created Shigella sonnei mutants lacking DNA polymerase I, impacting plasmid replication and increasing sensitivity to DNA damage. Revertants restored some enzyme activity and plasmid stability.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- DNA polymerase I is crucial for DNA replication and repair in bacteria.
- Shigella sonnei is a significant human pathogen.
- Understanding DNA repair mechanisms is vital for combating bacterial infections and antibiotic resistance.
Purpose of the Study:
- To isolate and characterize Shigella sonnei mutants deficient in DNA polymerase I.
- To investigate the role of DNA polymerase I in plasmid maintenance and DNA repair in S. sonnei.
- To compare S. sonnei DNA polymerase I mutants to their Escherichia coli counterparts.
Main Methods:
- Chemical mutagenesis of S. sonnei using nitrosoguanidine.
- Isolation of mutants using a plasmid (pBR313) that requires DNA polymerase I for replication.
- Phenotypic analysis of mutants for sensitivity to methyl methanesulfonate (MMS) and ultraviolet (UV) irradiation.
- Enzyme assays on crude extracts to confirm DNA polymerase deficiency.
- Isolation and characterization of MMS-resistant revertants.
Main Results:
- Successfully isolated S. sonnei mutants deficient in DNA polymerase I activity.
- These mutants were unable to maintain the pBR313 plasmid and exhibited increased sensitivity to MMS and UV irradiation.
- Enzyme assays confirmed the absence of significant DNA polymerase activity in the mutant extracts.
- MMS-resistant revertants showed partial restoration of DNA polymerase activity and improved plasmid stability.
- The observed properties of the S. sonnei mutants closely resemble those of E. coli polA mutants.
Conclusions:
- DNA polymerase I plays a critical role in plasmid pBR313 replication and DNA repair in Shigella sonnei.
- S. sonnei DNA polymerase I mutants share functional similarities with E. coli polA mutants, suggesting conserved roles.
- The study provides a foundation for further research into DNA repair pathways in S. sonnei and potential therapeutic targets.