Lecture 13 - Transposons, ICEs, PAIs

Question 1

Define transposons and describe their structure and the 3 types

Answer 1

• short DNA fragments with mechanism to move from 1 location to another
• transposase flanked by IRs or DRs
• IS elements, composite non-rep transposon, composite rep transposon

Question 2

Describe the 4 step process of non-rep transposition

Answer 2

1. TnpA transposase expressed from Tn gene
2. TnpA x2 binds flanking IR x2
3. TnpA cleaves DNA backbone, dimerises @ cut sites, loops out ICE element
4. TnpA recog attachment site elsewhere = nicks DNA and inserts IS via sstrand overhangs

Question 3

Describe the 5 step mechanism of rep transposition

Answer 3

1. TnpA sstrand cuts donor IRs and target site which fuse
2. sstrand copies as unstable cointegrates
3. Repair of ss to ds in both
4. Resolvase/TnpR = resolution @ res sites in transposon = homologous recombo between cointegrates
5. 2 circular DNA copies = complete transposon divided between the 2 dstrands

Question 4

What is cargo and how can transposons move it?

Answer 4

• fragments of genes adjacent to insertion sites
• transposases not super specific for IR cutting sites

Question 5

What are the 3 fates of movement by transposons?

Answer 5

1. another location in same genome
2. to conjugative plasmid to another cell
3. Compound transposon with acquired transfer function = ICEs

Question 6

What are ICEs and describe their structure

Answer 6

• Integrative and conjugative element
• IRs flanking integrase, excisionase, accessory and transfer genes
• dormant like prophase until excisionase triggered

Question 7

What are GIs?

Answer 7

• MGEs captured in genome - various sizes, foreign GC content, lost mobility by inactivated transposases/integrases, novel cargo regions

Question 8

What are PAIs?

Answer 8

• GIs with virulence/resistance genes converting commensal to pathogenic
• often adjacent to tRNAs

Question 9

How do bacteria evolve from commensal to pathogenic?

Answer 9

1. random mutation
2. MGEs
3. environmental selection

Question 10

What is EPEC and describe its pathogenic features/genes

Answer 10

• Enteropathogenic E coli (EPEC) = non-fatal infant diarrhoea
• T3SS syinge system by locus of enterocyte effacement (LEE) on PAI
• removes gut cilia for colonisation, EspC protease for water injection, EAF plasmid with bundle forming pilus (BFP) for adherence

Question 11

What is EHEC and describe its pathogenic features/genes

Answer 11

• Enterohaemorrhagic E coli (EHEC) = fatal toxigenic haemorrhagic colitis and haemolytic uremic syndrome
• LEE
• pO157 plasmid with protease and toxin and haemolysin to lyse RBCs
• StxO phage for heat stable/labile toxins for colitis
• rhamnose biosynthesis to modify LPS O antigen to endotoxin