African Trypanasomes

Question 1

What are they? What is their vector? Where do they live?

Answer 1

• single celled organisms which are highly motile via flagellum
• spread by tsetse flies
• are extra cellular, live outside of host cells

Question 2

Explain antigenic variation?

Answer 2

• 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

Question 3

What is VSG and what does it do?

Answer 3

• variable surface glycoprotein, which is a dense homogenous layer of protein on the parasite surface which allows antigenic variation

Question 4

How does VSG allow antigenic variation to occur?

Answer 4

• large masses of VSG coat entire parasite surface
• density shields the common proteins in membrane from being detected
• protein packs tightly arranged in dimmers

Question 5

How is VSG able to pack tightly?

Answer 5

• structure is conserved despite antigenic diversity, so protein has disulphide linkages which allow appropriate folding

Question 6

Where are VSG genes located?

Answer 6

• clusters at the very ends of chromosomes in subtelomeric locations
• also at very tip of telomeres within expression sites

Question 7

Properties of intermediate chromosomes?

Answer 7

• ~5 of them
• have VSG genes on each end
• comprised largely of repeat sequences so don’t have expression sure

Question 8

Properties of mini chromosomes?

Answer 8

• ~100 of them
• Have a VSG gene on each end
• rest of sequence is entirely repeats

Question 9

How are VSG genes expressed?

Answer 9

• 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

Question 10

How does the RNA pol 1 promoter work?

Answer 10

• Drives transcription through ESAGs, 70bp repeats, VSG gene and then terminates

Question 11

What is the RNA Pol1 promoter also known as?

Answer 11

Polycistronic transcription unit, as it comprises several genes encoded in the same pre mRNA

Question 12

What is the term RNA processing relating to?

Answer 12

The primary mRNA transcript has several genes encoded within it and need to get chopped up into gene size bits

Question 13

Why do trypanosomes use RNA Pol I instead of RNA Pol II?

Answer 13

• 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

Question 14

How are silent VSG genes expressed?

Answer 14

• Genes in silent subtelomeric locations or on other chromosomes must relocate to active exoression site

Question 15

What is duplicative transposition?

Answer 15

• 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

Question 16

How are genomic VSG genes pseudogenes?

Answer 16

Have stop codons and frame shift, cannot encode an entire functional protein

Question 17

What are mosaic VSG genes?

Answer 17

Incomplete VSG genes are combined to form a functional intact one to increase number of potential VSG variants

Question 18

What is the order of VSG expression hierarchy?

Answer 18

• 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

Question 19

When are mosaic genes detected?

Answer 19

Late in chronic infection when antibodies have been generated against all other VSG types

Question 20

VSG sequence dependency in stage of infection?

Answer 20

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

Question 21

Explain differences between slender and stumpy variants?

Answer 21

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

Question 22

How does the parasite know when to transform?

Answer 22

• generates and responds to signal stumpy induction factor (SIF)
• release SIF into environment, and use signal as density sending information

Question 23

How do parasites transform from slender to stumpy?

Answer 23

• 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

Question 24

How does stumpy form restrict antigenic variation rates?

Answer 24

Stumpy forms cannot undergo antigenic variation as they are irreversibly committed to cell cycle arrest

Question 25

Why is stumpy form crucial for trypanasome dynamics?

Answer 25

• prolongs host survival
• sustains transmission
• ensures use of antigen repertoire