African Trypanasomes Flashcards
What are they? What is their vector? Where do they live?
- single celled organisms which are highly motile via flagellum
- spread by tsetse flies
- are extra cellular, live outside of host cells
Explain antigenic variation?
- In the first wave, parasites are growing and presenting particular antigen types
- host raises antibodies against these antigens and kills the parasite
- infection isn’t completely killed as some parasites change expression to an antigenically distinct surface coat
- new parasites grow and repeat the process
What is VSG and what does it do?
- variable surface glycoprotein, which is a dense homogenous layer of protein on the parasite surface which allows antigenic variation
How does VSG allow antigenic variation to occur?
- large masses of VSG coat entire parasite surface
- density shields the common proteins in membrane from being detected
- protein packs tightly arranged in dimmers
How is VSG able to pack tightly?
- structure is conserved despite antigenic diversity, so protein has disulphide linkages which allow appropriate folding
Where are VSG genes located?
- clusters at the very ends of chromosomes in subtelomeric locations
- also at very tip of telomeres within expression sites
Properties of intermediate chromosomes?
- ~5 of them
- have VSG genes on each end
- comprised largely of repeat sequences so don’t have expression sure
Properties of mini chromosomes?
- ~100 of them
- Have a VSG gene on each end
- rest of sequence is entirely repeats
How are VSG genes expressed?
- only expressed in expression sites of mega base chromosomes, ~22 sites in total
- only 1 VSG gene is expressed at a time, so only 1 expression site is active
How does the RNA pol 1 promoter work?
- Drives transcription through ESAGs, 70bp repeats, VSG gene and then terminates
What is the RNA Pol1 promoter also known as?
Polycistronic transcription unit, as it comprises several genes encoded in the same pre mRNA
What is the term RNA processing relating to?
The primary mRNA transcript has several genes encoded within it and need to get chopped up into gene size bits
Why do trypanosomes use RNA Pol I instead of RNA Pol II?
- the trypanasome needs lots of VSG RNa made in order to make enough VSG protein to coat entire parasite
- uses highly active promoter instead to speed process
How are silent VSG genes expressed?
- Genes in silent subtelomeric locations or on other chromosomes must relocate to active exoression site
What is duplicative transposition?
- clusters of silent VSG genes have stretches of 70bp repeats which act as catalyst for gene conversion
- gene within active expression site is deleted, while gene used to replace is duplicated
How are genomic VSG genes pseudogenes?
Have stop codons and frame shift, cannot encode an entire functional protein
What are mosaic VSG genes?
Incomplete VSG genes are combined to form a functional intact one to increase number of potential VSG variants
What is the order of VSG expression hierarchy?
- the VSG genes expressed are intact VSGs within expression sites
- activation of VSG genes in subtelomeric locations which need to be moved to active site via gene conversion
- activation of VSG genes requiring assembly of mosaic genes from bits
When are mosaic genes detected?
Late in chronic infection when antibodies have been generated against all other VSG types
VSG sequence dependency in stage of infection?
Early infection: not dependent on sequence, is dependent on 70bp repeats which flank the VSG gene
Late infection: dependent on sequence, as genes may only be activated through recombination with gene depending on coding sequence
Explain differences between slender and stumpy variants?
Slender: divide, responsible for ascending number of parasites in each wave, stop dividing to avoid killing cell
Stumpy: not dividing, dominates at wave peak when slender form stops dividing
How does the parasite know when to transform?
- generates and responds to signal stumpy induction factor (SIF)
- release SIF into environment, and use signal as density sending information
How do parasites transform from slender to stumpy?
- parasite releases peptidases which act on external protein in the blood
- peptidases degrade proteins into fragments called oligopeptides
- oligopeptides tell parasite density
- large numbers of oligopeptides activate signalling pathway that causes slender stumpy transition
How does stumpy form restrict antigenic variation rates?
Stumpy forms cannot undergo antigenic variation as they are irreversibly committed to cell cycle arrest
