trans

POST-TRANSCRIPTIONAL REGULATION IN OTHER PARASITES 85
deadenylation or endonuclease cleavage) are
complete.
Control of translation and protein
processing
Translational control is probably extremely
important in kinetoplastids, but has until now
been almost completely neglected. Investigators
looking for regulated genes have adopted
two main approaches. They have cloned genes
encoding proteins whose expression was
already known to be regulated, such as enzymes
involved in energy metabolism. Alternatively
they have looked for RNAs that were preferentially
expressed in one or other life-cycle stage
by differential screening of libraries or, more
recently, using PCR-based approaches. Overall
this has resulted in a bias towards genes whose
products were abundantly expressed and regulated
at the mRNA level. Nevertheless indications
of translational control have emerged.
The control of EP and HSP translation has
already been mentioned. More dramatically,
cytochrome c and aconitase are 30–100 times
more abundant in procyclic T. brucei than in
bloodstream forms, but the differences in
mRNA are very small. The PGK genes may also
show different types of regulation in different
trypanosome strains. The results so far
described have been from a highly virulent,
laboratory-adapted strain of parasites, but
in another strain, most regulation seemed to be
of protein synthesis rather than of the mRNA
level.
Several trypanosomatid RNAs have long 5untranslated
regions which include upstream
open reading frames which might be expected
to drastically reduce translation of the major
long open reading frame. One example is
the PAG genes, located downstream of the
EP/GPEET genes, but it is not known if
the proteins predicted for these genes are
produced at all. Another is the tuzin 5untranslated
region from T. cruzi, which has an
upstream ORF which suppresses translation.
Protein degradation
A discrepancy between mRNA levels and protein
levels can be caused not only by regulation
of translation, but also by differential
protein degradation. For mitochondrial proteins
like cytochrome c and aconitase, a failure
of protein import could lead to degradation;
so far, though, there is no evidence that bloodstream
trypanosome mitochondria are defective
in protein import. Similarly, one could
imagine that differences in the secretory pathway
such as regulated expression of modification
enzymes might result in failure to correctly
process a protein and consequent degradation.
The proteasome of T. brucei contains, like
other proteasomes, a catalytic core of about
20 S which is inhibited by lactacystin. Treatment
of both procyclic and bloodstream trypanosomes
with lactacystin resulted in cell cycle
arrest, confirming that proteasome activity is
needed for trypanosome growth. Two proteins
which may be proteasome substrates are CYC3
and CYC2. These two cyclins have half-lives of
less than one cell cycle, and their stability is
increased by proteasome inhibitors.
POST-TRANSCRIPTIONAL
REGULATION IN OTHER
PARASITES
There is extraordinarily little information available
about post-transcriptional regulation
in parasites other than the Kinetoplastida.
Work on the apicomplexans has concentrated
almost exclusively on the search for transcriptional
promoters and analyses of transcriptional
regulation. This is doubtless partially for
MOLECULAR BIOLOGY