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Mol. Cell. Biol., Feb 1997, 833-843, Vol 17, No. 2
R McCulloch, G Rudenko and P Borst
African trypanosomes undergo antigenic variation of their variant surface
glycoprotein (VSG) coat to avoid immune system-mediated killing by their
mammalian host. An important mechanism for switching the expressed VSG gene
is the duplicative transposition of a silent VSG gene into one of the
telomeric VSG expression sites of the trypanosome, resulting in the
replacement of the previously expressed VSG gene. This process appears to
be a gene conversion reaction, and it has been postulated that sequences
within the expression site may act to initiate and direct the reaction. All
bloodstream form expression sites contain huge arrays (many kilobase pairs)
of 70-bp repeat sequences that act as the 5' boundary of gene conversion
reactions involving most silent VSG genes. For this reason, the 70-bp
repeats seemed a likely candidate to be involved in the initiation of
switching. Here, we show that deletion of the 70-bp repeats from the active
expression site does not affect duplicative transposition of VSG genes from
silent expression sites. We conclude that the 70-bp repeats do not appear
to function as indispensable initiation sites for duplicative transposition
and are unlikely to be the recognition sequence for a sequence-specific
enzyme which initiates recombination-based VSG switching.
Copyright © 1997, American Society for Microbiology
Gene conversions mediating antigenic variation in Trypanosoma brucei can occur in variant surface glycoprotein expression sites lacking 70- base-pair repeat sequences
Division of Molecular Biology, The Netherlands Cancer Institute, Amsterdam.
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