trans

POST-TRANSCRIPTIONAL REGULATION IN KINETOPLASTIDS 73
regulated by post-translational modifications
such as phosphorylation, addition of fatty acid
chains, glycosylation, and specific internal
cleavage. Some proteins are needed only transiently
in the cell. Some are recognized by
specific proteases, but more often they are
destroyed by a general protease complex, the
proteasome. Proteins destined for degradation
bear specific recognition sequences which are
recognized by enzymes that transfer a small
protein, ubiquitin, onto the protein (ubiquitinconjugating
enzymes). After this, further
ubiquitin is added. The ubiquitin chain is recognized
by the proteasome. To regulate degradation,
the recognition sequences may, for
instance, be concealed by modification or
interaction with other proteins. All of this is
vital to growth and survival.
POST-TRANSCRIPTIONAL
REGULATION IN
KINETOPLASTIDS
Structure and transcription of
kinetoplastid genomes
The genome structure and transcription patterns
of kinetoplastids are unique in evolution.
Instead of having one strictly controlled
transcriptional promoter per gene (or operon)
most genes appear to have no specific transcriptional
promoter at all. This is illustrated
nicely by Leishmania major chromosome I:
this chromosome has a short segment lacking
open reading frames, surrounded on one side
by 27 genes that are arranged head-to-tail and
oriented towards one telomere, and on the
other side by 50 genes that are oriented towards
the other telomere. There could perhaps be
transcriptional promoters in this central region,
but that still would mean that all the genes are
transcribed all the time.
There is abundant evidence for polycistronic
transcription in kinetoplastids. Precursor RNAs
containing two open reading frames, and bridging
across from one 3-untranslated region to
the next 5-untranslated region, have been
seen by Northern blotting and by reversetranscription
and PCR. Transcription assays
have shown that transcription of individual
genes remains constant irrespective of the
abundance of the mRNA product. The only
known examples of transcriptional control are
found in the salivarian trypanosomes, where
the major surface protein genes (VSG and the
EP/GPEET family) and associated genes are
transcribed by RNA polymerase I and the transcriptional
activity is regulated by chromatin
structure (see Chapters 3 and 5).
The need for post-transcriptional
regulation
Despite their lack of transcriptional control, the
kinetoplastids must adapt their gene expression
to a changing environment. They have various
life-cycle stages – mostly spindle-shaped and
flagellate, but, for amastigotes, spherical and
aflagellate. Most of the organisms of human
and veterinary importance are digenetic, which
means that they have two hosts – the mammal
and an arthropod. In the mammal, the parasites
must evade the humoral immune response.
For Trypanosoma cruzi and Leishmania
amastigotes this means adjusting to intracellular
life in the cytosol and lysosome respectively,
and for T. cruzi trypomastigotes, extracellular
survival as well. For the salivarian trypanosomes,
such as the bloodstream-form
trypomastigotes of T. brucei, it means perpetual
variation of the antigenic surface coat
(variant surface glycoprotein or VSG). The salivarian
trypanosomes also have a non-dividing
mammalian stage, the stumpy form, which is
pre-adapted for initial survival in the insect
MOLECULAR BIOLOGY