trans

86 POST-TRANSCRIPTIONAL REGULATION
technical reasons, as promoter activity can
be assayed in transient transfection assays,
whereas work on RNA processing and turnover
requires much larger amounts of parasite
material, derived from permanent transformants.
Many apicomplexan genes contain introns.
Potential splicing signals have the features of
canonical eukaryotic signals, but have not
been functionally tested. Nevertheless, at least
two possible examples of alternative or regulated
splicing have been documented. A study
of the B7 gene of Plasmodium berghei showed
that the transcripts expressed in the asexual
and sexual stages differed in the length of the
5-untranslated region because two different
promoters and alternative splice sites were
used, and analysis of the MSP4/5 genes of
P. berghei and Plasmodium yoelii revealed that
both unspliced and spliced mRNA were present
at steady state.
Just as for splicing, no polyadenylation
signals have been functionally identified in
Apicomplexa. The canonical mammalian
polyadenylation signal, AAUAAA, occurs many
times by chance in the AU-rich Plasmodium
falciparum genome; in a random sequence
containing 50% A and 50% T, the sequence
would occur by chance once every 64 nt. Thus
any assignment of functionality to this
sequence has to be treated with extreme caution.
For example, a 750 nt region 3 of the pgs28
coding region of Plasmodium galinaceum was
found to contain seven AAUAAA or AUUAAA
signals, of which five were contained in the
3-untranslated region of the mRNA and two
were further downstream. Deletion of either
a 155 nt poly(T) tract upstream of the fifth
signal, or of the region downstream of the
poly(T) tract including the fifth signal, reduced
reporter gene expression in transient assays by
about 90%. But it is not known whether the
deletions affected the polyadenylation site. RNA
degradation and translational control have not
been studied in Apicomplexa and no examples
of 3-untranslated region-mediated regulation
are known. Regulation of RNA processing, stability
and translation in other parasites, whether
unicellular or multicellular, has received almost
no attention. This field is therefore wide open
for future work.
POST-TRANSCRIPTIONAL
REGULATION AND
ANTI-PARASITIC
CHEMOTHERAPY
The vital importance of post-transcriptional
regulation in the kinetoplastids might suggest
this process as a possible target for future antikinetoplastid
drugs. Obviously, if the regulation
were inhibited, the whole metabolism
and surface structure of the organisms would
become deregulated, with lethal consequences.
Thus an African trypanosome might not only
lose control over its energy metabolism, but
also start to express the EP and GPEET surface
proteins constitutively, making it a ready target
for the immune response. Before any new
process can be specifically inhibited, however,
it is necessary to identify the proteins involved
and to define their mechanism of action.
Even once this is achieved, it is not clear that
post-transcriptional regulation is a good target.
Many of the nucleases are conserved in
evolution. To attack the direct regulatory proteins
it would probably be necessary to inhibit
protein–RNA interactions. It is sobering to
note that although post-transcriptional regulation
is of vital importance in the regulation
of the immune response, and possibly in growth
of cancer cells, so far no specific inhibitors of
this process are (to my knowledge) available.
If the RNA degradation and stabilization
machineries are not amenable to selective
MOLECULAR BIOLOGY