3. HGT and bacterial evolution Flashcards
Differentiate between point mutations and horizontal gene transfer
Point mutations - slow because small fragments incorporated - duplication + change
HGT - rapid because large fragments incorporated
What genetic elements are involved in bacterial evolution?
- bacteriophages
- conjugative plasmids
- conjugative and mobilisable transposons
What are the mechanisms of HGT?
Mechanisms of HGT:
1.1 Transduction - bacteriopahge lysogeny and transduction
1.2 Conjugation of plasmid - physical interaction needed
1.3 Transformation via naked DNA in the environment
2. Restriction
3. Integration into host chromosome
Explain vertical vs horizontal gene transfer
Why is HGT powerful in bacterial evolution?
Because bacteria:bacteriophage ratio is 1:10 - so many bacteriophages for HGT
How can foreign DNA be incorporated into host genome?
Mechanisms of foreign DNA incorporation into host genome:
- homologous recombination
- site-specific recombination
Explain when and how HR occurs in foreign DNA incorporation into host genome
For HR:
- foreign sequence has high level of homology (>70%) - DNA from closely related species
- length ~100 nt
- 2 recombination events to inoporate foreign linear into host circular - to maintain circularity
- host original sequence deleted -> result: increase / decrease in genome size depending on foreign DNA length
Explain when and how site-specific recombination occurs in foreign DNA incorporation into host genome
For site-specific recombination:
- foreign sequence has low level of homology - DNA from distantly related species
- length ~30 nt
- 1 recombination event to incorporate the circular foreign into host’s circular chromosome
- host sequence remains + inserted foreign DNA -> result: always increase in genome size
How does pathogenicity evolve?
Through HGT of pathogenic genes - incorporation of pathogenicity islands (PAI)
What is the structure of a pathogenicity island?
Pathogenicity island (PAI)
- commonly inserted near / into tRNA genes -> tRNA gene parts flank PAI
- direct repeats
- integrase (int) gene
- pathogenicity related ORF
Pathogenicity islands have lower GC% than host’s core DNA - can identify PAI
What are genomic islands?
Genomic island example - pathogenicity islands (PAI)
What are the enzymes used in site-specific recombination?
- resolvase / invertase - catalytic serine - OH attack
- integrase - catalytic tyrosine - OH attack
What determines the orientation of an insert in site-specific recombination?
Outcome depends on orientation of sites:
- inverted repeats: sequence inversion
- direct repeats: sequence excision / integration
What is the recombination mechanism of resolvase / invertase?
- 2 recombinases attach to target site - form synapse - minimum 2** bases of homology** needed
- Recombinases make 4 cleavages on the 4 strands and rotate the sequences in right-handed rotation
- Re-ligation of the strands => recombinant products
No ATP used - E from supercoiling used
Transesterification
What is the recombination mechanism of integrase?
- 4 integrases for 2 dsDNA strands - cleavage of 2 strands (1 on each)
- Attack - Holliday junction formed
- Resolving HJ - cleavage + transesterification
What is the main barrier to HGT?
When DNA introduced through HGT - restriction used to degrade - protection from integration
How do bacterial cells protect againts bacteriophage infection?
By active restriction-modification system / CRISPR-Cas9: unmethylated foreign DNA recignised - attacked - degradation of phage DNA
Lecture summary
