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E.coli systems and recombination: Determinants of diversity: Overall aims ML

E.coli systems and recombination: Determinants of diversity: Overall aims ML. Nine/ten lectures with Key topics. Homologous recombination and DNA repair Role of methylation and repair. Role of Plasmids; control of replication, transfer and stability.

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E.coli systems and recombination: Determinants of diversity: Overall aims ML

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  1. E.coli systems and recombination: Determinants of diversity: Overall aims ML • Nine/ten lectures with Key topics. • Homologous recombinationand DNA repair • Role of methylation and repair. • Role of Plasmids; control of replication, transfer and stability. • Illegitimate recombination: transposons and integrons • Regulation of DNA transposition. • You should: • Have a basic grounding for further reading and other systems covered in the course (e.g pathogens). • Be able to critically read key papers in the area. • Critically assess the development of ideas to date.

  2. DNA Repair mechanisms • This section will review: • The Role of DNA damage and its repair in the generation of genetic diversity in bacteria • The mechanistic links of repair with recombination systems covered earlier • You should be able to discuss the effect of environment on damage and repair. For example: • Chemical and Radiation effects • Phagocytic damage in relation to pathogen survival in the host • Errors in replication and their repair.

  3. How can DNA become damaged? • Mismatched bases • Polymerase error rate about 1 in 104 (see later lectures) • Deamination of C to U leading to mismatch • Missing bases. Hydrolysis of purine-deoxyribose bond leading to AP-site. • Structural damage. Dimer formation. • Broken phosphodiester bonds. Chemicals/radiation • REPAIR MECHANISMS NECESSARY FOR SURVIVAL.

  4. C A T G Mismatch ds DNA Break C-U deamination ss Break AP site Covalent X-linking Thymidine dimer Types of DNA Damage Summarised

  5. General repair mechanisms needed • EITHER Reverse damage (e.g. PHOTOREACTIVATION) • OR excise DNA and patch repair the region Photoreactivation: Discovered in Actinomycetes in 1949 UV - DNA Damage - Cell Death UV - Bright visible light - survival ! • 3 Steps: • Photolyase (encoded by phrA and phrB genes in E. coli) recognises distortion at dimer. • Light activates photolyase • Dimer cleaved

  6. Repair- UV WT Repair T-Phage suspension Plaques ! General repair mechanisms needed: Excision repair. • Discovered first as a general mechanism in 1964 • T- Phage HOST CELL REACTIVATION DNA Damage

  7. UvrABC Pol1 5’ 5’ 5’ 5’ 5’ 3’ 3’ 3’ 3’ 3’ 5’ 5’ 5’ 5’ 5’ 3’ 3’ 3’ 3’ 3’ 5’ 3’ 5’ 3’ 5’ 3’ 5’ 3’ Excision repair........ OR

  8. Other repair routes. • Excision repair involves up to 20 nucleotides • uvrA,BC (D) mutants very senstive to UV light • Mismatch repair • A from of excision repair. Dam methylase involved • see later re: methylation • N-glycolylase excision repair • Uracil either misincorporated OR C deaminated to U • Uracil N-glycosylase action TO give AP site • AP endonuclease cut • Patch repairs above

  9. How does UV light cause mutations? Discovery of error-prone repair. • RecA required for high level of UV mutagenesis • UV dose of 2 µJ/mm2 leads to 120 dimers • Long patch Error-prone repair • Post dimer initiation • Trans dimer synthesis The SOS Hypothesis Radman 1974 originally proposed an inducible repair system Requires RecA and a regulator system

  10. SOS System in E.coli • Repair normally at low level • lexA gene identified as a regulator • Recombine normally • But NO increased UV mutagenesis (ie 30 dimers produces no extra mutants). Higher doses required • LOW DOSE - Error-free repair • HIGH DOSE - Error repair INDUCED • LexA is an autoregulated repressor • Represses level of activity of many genes • Collectively called DNA Inducible (din) genes • Includes uvrA,B,C,D and sfi etc... • RecA protease activity; Cleaves LexA • Also CI repressor inducing lysis

  11. lexA lexA recA recA PO PO PO PO din din din din din din din din din din PO PO PO PO PO PO PO PO PO PO DNA inducible repair genes SOS System in E.coli Low level expression HIGH level expression

  12. Post Replication / Recombination Repair • recA mutants VERY UV sensitive • uvrA similarly • uvrA recA mutants VERY VERY UV sensitive • recA mutants recover slowly in the dark • uvrA mutants do not UV dose Dimers/ Genotype Phenotype µJ/mm2 genome WT WT 5.0 3200 uvrA No excision repair 0.8 50 recA No recombination 0.3 20 recA, uvrA Neither 0.02 1

  13. Post Replication / Recombination Repair • Stephen C. Kowalczykowski (2000) Initiation of genetic recombination and recombination-dependent replication. TIBS 25 – April 2000 • Double-stranded-break repair – see transposition of Tn10 later • Recombination-dependent replication • Replication-dependent recombination • SEE ALSO Key Reference in NATURE; STRUCTURE OF RecBCD complex • Nature 11th Nov 2004 vol 432, 187-193

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