Figure 1.
VSG Switching Hierarchy in T. brucei
The graph is adapted from [9] and shows the numbers of T. brucei cells (parasitaemia) measured in a cow for up to 70 days post-infection (this measurement is depicted by inversely plotting the prepatent period, in days, that a 0.2-ml inoculum of cattle blood achieves a parasitaemia of 1 × 108.1 trypanosomes ml−1 units in an immunosuppressed mouse). Below the graph is a depiction of VSG gene activation timing (see Figure 2 for details of the switch mechanisms). During VSG switches driven by recombination, silent VSGs at a telomere are, in general, activated more frequently that subtelomeric array VSGs, which are activated more frequently than VSG pseudogenes (pseudo). It is unclear (indicated by a question mark) if transcriptional switches between VSG bloodstream expression sites (BES) occur predominantly at the start of an infection or continue throughout.
Figure 2.
Mechanisms of VSG Switching during Antigenic Variation in T. brucei
The VSG gene expressed prior to a switch (indicated by a blue box) is transcribed from an expression site (ES) that is found at the telomere (vertical black line) of a chromosome (horizontal black line); active transcription of the ES is indicated by a dotted arrow, ESAGs are depicted by black boxes, and 70-bp repeat sequence is shown as a hatched box. Gene conversion to generate a VSG switch can occur by copying a silent VSG (red box) from a subtelomeric array into the ES, replacing the resident VSG; the amount of sequence copied during gene conversion is illustrated, and normally encompasses the VSG ORF and extends upstream to the 70-bp repeats. The silent VSG donor can also be telomeric (either in a mini chromosome or in an inactive ES); here, the downstream limit of conversion can extend to the telomere repeats, while the upstream limit can either be in the 70-bp repeats or the ESAGs (if the donor is in an ES). Segmental VSG conversion involves the copying of sequence from multiple, normally nonfunctional VSGs (pink, red, or green boxes) to generate a novel mosaic VSG in the ES. In transcriptional VSG switching, recombination appears not to be involved; instead, limited transcription at a silent VSG ES (indicated by a small arrow) becomes activated to generate fully active transcription, while the previously active ES is silenced.
Figure 3.
Functions of Dot1-Mediated Histone H3 Methylation
Two models for the role of Dot1-mediated methylation of histone H3 are diagrammed, comparing a Dot1 mutant (ΔDot1) and a wild-type cell. The repulsion model is derived from [21]. Methylation of histone H3 is indicated by “me”, and the level of transcription of a chromosome (black line) is indicated by a shaded gray bar (a thick bar indicates active transcription, a thin bar indicates silenced transcription). Silencing factors (such as Sir proteins in yeast; light blue circles) are indicated localised to the telomere (vertical line) in wild-type cells, being excluded from elsewhere by H3K79 methylation. Mutation of Dot1 removes H3K79 methylation, de-repressing transcription of the telomeric region. The recruitment model is based on [32], and shows the same region of chromosome after suffering a DNA double-strand break (gap in the line). Here, histone H3K79 methylation recruits a checkpoint signalling factor (Rad9 in yeast; dark blue circle), and in the absence of histone H3K79 methylation processing of the DNA break to yield single stranded DNA is increased, amplifying the DNA damage signalling cascade.