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Microbiol. Rev., Jun 1995, 223-240, Vol 59, No. 2
L Vanhamme and E Pays
Trypanosomes are protozoan agents of major parasitic diseases such as
Chagas' disease in South America and sleeping sickness of humans and nagana
disease of cattle in Africa. They are transmitted to mammalian hosts by
specific insect vectors. Their life cycle consists of a succession of
differentiation and growth phases requiring regulated gene expression to
adapt to the changing extracellular environment. Typical of such
stage-specific expression is that of the major surface antigens of
Trypanosoma brucei, procyclin in the procyclic (insect) form and the
variant surface glycoprotein (VSG) in the bloodstream (mammalian) form. In
trypanosomes, the regulation of gene expression is effected mainly at
posttranscriptional levels, since primary transcription of most of the
genes occurs in long polycistronic units and is constitutive. The
transcripts are processed by transsplicing and polyadenylation under the
influence of intergenic polypyrimidine tracts. These events show some
developmental regulation. Untranslated sequences of the mRNAs seem to play
a prominent role in the stage- specific control of individual gene
expression, through a modulation of mRNA abundance. The VSG and procyclin
transcription units exhibit particular features that are probably related
to the need for a high level of expression. The promoters and RNA
polymerase driving the expression of these units resemble those of the
ribosomal genes. Their mutually exclusive expression is ensured by controls
operating at several levels, including RNA elongation. Antigenic variation
in the bloodstream is achieved through DNA rearrangements or alternative
activation of the telomeric VSG gene expression sites. Recent discoveries,
such as the existence of a novel nucleotide in telomeric DNA and the
generation of point mutations in VSG genes, have shed new light on the
mechanisms and consequences of antigenic variation.
Copyright © 1995, American Society for Microbiology
Control of gene expression in trypanosomes
Department of Molecular Biology, Free University of Brussels, Rhode Saint Genese, Belgium.
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