trans

GENETICS OF ANTIGENIC VARIATION 97
its efficient secretion. The trypanosome appears
to monitor both the overall structure of VSG,
perhaps through a specific chaperone molecule,
and the presence of a GPI anchor. Both
components are necessary for efficient
exocytosis. Further studies of trypanosomes
may illuminate the general roles of GPI
anchors in protein sorting, secretion and
endocytosis.
GENETICS OF ANTIGENIC
VARIATION
Each VSG is encoded by a unique gene.
Variation is usually not generated by contemporaneous
recombination events between gene
segments, or by fusion of constant and variable
regions. Instead, the genome of T. brucei
contains hundreds of competent VSG genes,
together with a large number of pseudogenes
that can nevertheless contribute, through
recombination events, to the expressed repertoire.
The repertoire is probably evolving and
expanding continuously. Although some VSG
genes are conserved between different trypanosome
isolates, others are unique to particular
clones, and the total potential VSG
repertoire that exists in the field may be essentially
infinite.
If there are hundreds of VSG genes, how does
the trypanosome determine that only one VSG
appears on the cell surface? Part of the answer is
that the expressed VSG is always located (or
relocated) close to a telomere, in a polycistronic
transcription unit that we call an expression site
(ES). This appears to be a necessary but insufficient
condition for the singularity of VSG expression,
because there are several ESs, with very
similar structures, that can be turned on and off
without discernable changes in sequence or
organization. The question then becomes why
is only one ES transcribed at any time? This system
raises many of the same issues and also represents
an interesting paradigm for the general
question of how many kinds of cells regulate
the expression of one among many similar
alleles. In no case is this kind of allelic selectivity
completely understood. In mammals there
are many examples of allelic exclusion, but
ES switching may be a unique example of
reversible allelic exclusion, perhaps because
try- panosomes are continuously dividing,
whereas allelic exclusion is generally an irreversible
commitment that occurs at a particular
stage of development or cellular differentiation.
Olfactory receptors, which are encoded by the
largest family – 1000 genes – in mammals, are
perhaps the most extensive example of allelic
exclusion, with each olfactory neuron expressing
a single receptor gene.
There are two well characterized classes of
VSG expression sites, which differ dramatically
in their organization and which operate
in the two different infectious stages of the
trypanosome life cycle. Both bloodstream and
tsetse metacyclic ESs contain a VSG in the
most telomere-proximal position, about 1–3 kb
upstream of the hexanucleotide (TTAGGG)
telomeric repeats. The ESs used in the bloodstream
stage are transcribed from a promoter
that is typically 30–50 kb upstream of the VSG,
and 10–50 kb tracts of a conserved ‘50-bp
repeat’ are found upstream of ES promoters
(Figure 5.4). Between the promoter and the VSG,
ESs typically contain eight to eleven ‘expressionsite-associated
genes’ (ESAGs). Some ESAGs are
present in multiple copies in some ESs. Some
ESAGs appear to be truly unique to ESs, but
others are members of larger families that are
also found outside the ES (Table 5.1). ESs can
also contain fragmentary remnants of ESAG
sequences. In contrast, metacyclic ESs appear
to be monocistronic: the promoter is immediately
upstream from the telomeric VSG. All ESs
MOLECULAR BIOLOGY