trans

GENETICS OF ANTIGENIC VARIATION 101
structure having an intrinsic suppressive effect
on transcription, perhaps because telomeres
are sequestered in particular regions of the
nucleus (specifically at the nuclear envelope
in S. cerevisiae). However, it is unclear what
would allow just one telomere to escape a suppressive
structure. TPE has been demonstrated
in T. brucei, by inserting reporter cassettes into
different genomic environments, but it is not
clear how much of a role it has in regulating ES
transcription from the telomere-distal ES promoter.
TPE could play a role in suppressing transcription
from telomere-proximal metacyclic
ES promoters.
The use of trypanosome lines that have been
highly adapted to laboratory use has received
criticism from some investigators, on the basis
that these lines have been selected for virulence
and VSG stability, which may be incompatible
with, and have led to loss or attenuation of
mechanisms involved in, rapid VSG switching.
On the other hand, most studies of VSG switching
would not have been successful in rapidly
switching lines. Because of this issue, suggestions
about the relative roles of different switching
mechanisms should be not be considered
to be written in stone, at this stage of our knowledge.
The existence of specific VSG recombination
mechanisms is suggested by the extremely
conserved sequences flanking the VSG conversion
cassette, which form the boundaries
of the gene conversion in most cases in which
these have been determined. Again, there are
exceptions to this statement, and one must
always carefully examine the experimental conditions
used in different studies before making
direct comparisons of the outcome and
interpretation.
When metacyclic trypanosomes enter the
bloodstream, they continue to express metacyclic
VSGs for several cycles of replication.
Then the metacyclic ESs shut down and a
bloodstream ES takes over. It has been
suggested that the early bloodstream infection
is a time of rapid VSG switching. Both logic and
experimental evidence suggest that gene conversion
is likely to be the major mechanism for
VSG expression, because there are so many
silent VSGs and only a few ESs. However, most
mechanistic studies of VSG switching have so
far focused on ES switching, probably because
it is more accessible to experimental investigation.
One interesting question is why multiple
ESs exist? One possibility is that it is not only
the VSG that needs to be exchanged on the trypanosome
surface. Several ESAG-encoded proteins
are predicted to be present on the cell
surface (Table 5.1). Perhaps these proteins have
specific functions that make them less tolerant
of variation, or perhaps they are less exposed to
the immune response, so the existence of only a
few variants, encoded in different ESs, is sufficient
for their survival and function. This may
be fortunate because the existence of two large
families of variant proteins, whose switching
had to be coordinated by simultaneous recombination
events, would pose a major problem
for any cell. The function of only two ESAGs is
known: the proteins encoded by ESAG6 and
ESAG7 form a heterodimeric receptor for transferrin,
whose uptake is essential for the survival
of bloodstream trypanosomes. Several studies
have shown that the transferrin receptors
encoded by different ESs have different affinities
for transferrins from different host species.
It has been suggested that this variation in
transferrin receptor affinity, perhaps coupled
to other receptor interactions, could allow
T. brucei to optimize its growth in different hosts,
and this would determine which ES would be
favored in any particular host. Although this is
an interesting hypothesis, it remains a hypothesis.
One interesting point about the transferrin
receptor is that it is encoded by the most promoter-proximal
ESAGs, and these are transcribed
simultaneously at a low level from many
MOLECULAR BIOLOGY