Prokaryotes: Genome Evolution Flashcards
What causes problems in assembling genomes?
Repetitive sequences
How does evolution occur in bacteria?
Point mutations - slow
Horizontal gene transfer - fast
Gene duplication is important and so point mutations are slow
What genetic elements are involved in HGT?
Bacteriophages
Conjugative plasmids
Conjugative & mobilisable transposons
What mechanisms of HGT are there?
Lysogeny and transduction (phages)
Conjugation
Transformation - if bacteria naturally competent
Need to survive restriction enzymes
How many bacteriophages per bacterium on average?
10 for every 1
How is foreign DNA incorporated into bacterial genomes?
Homologous recombination (recA) for linear DNA - closely related strains/species (needs homologous sequences ) Site-specific recombination for circular DNA - distantly related species
How has E. coli evolved pathogenicity?
K-12 commensal:
Acquisition of plasmids (F plasmid)
Lambda phage integration
K islands
EHEC:
O islands
STX phage - toxins
LEE
Uropathogenic:
Pathogenicity islands in many locations
What qualities do genomic island have?
Horizontally acquired Large inserts Sequence composition different from core backbone Insertion adjacent to/within tRNA genes Flanked by direct repeats Recombinase genes Genetic instability Often mosaic structures of several individual acquisitions
What are the 2 main families of bacterial site-specific recombinases?
Resolvase/Invertase (catalytic residue = serine)
Integrase (catalytic residue = tyrosine) (Xerc and XerD)
How does the orientation of sites impact the outcome of site specific recombo?
If opposite directions then cause inversion
If same direction cause excision/resolution and integration/fusion
What are integrons?
DNA elements which can capture and carry genes
Important for generation and transfer of multiple resistances
Often in compound transposons and conjugative plasmids
What are the 3 types on restriction enzymes?
Type I: endonuclease (1 gene) and methyl transferase (2 genes) don't interact only cleaves unmethylated sequences Type II: ...
What is the pan-genome?
The core genes + the dispensable genes
As new strains sequenced in general more genes are found
How are phage-encoded virulence factors integrated?
Lysogeny
What do we know about Corynebacterium diptheriae
Gram +ve
Colonice upper respiratory tract
Release DT (toxin) encoded by tox gene on the integrated beta-phage
PAIs encode majority of fimbrial & fimbria-related genes, iron-uptake systems, and lantibiotic biosynthesis system
