trans

SL RNA AND U snRNA GENE TRANSCRIPTION IN NEMATODES AND TRYPANOSOMATIDS 59
Core promoters of Toxoplasma gondii genes
apparently lack TATA boxes. However, in
NTPase genes an initiator element was identified
which is essential for efficient reporter
gene expression. The initiator sequence is
similar to that of higher eukaryotes and is also
present in the surface antigen gene 1 (SAG1).
Further analysis of the NTPase 3 gene promoter
showed that cis-activators are present
upstream of position 141. NTPase gene
expression is downregulated upon differentiation
from the tachyzoite to the bradyzoite
stage. Interestingly, life-cycle-specific regulation
was still observed with a promoter 5
truncated to position 141, indicating that it
contains a determinant for the developmental
control of promoter activity. The SAG1 gene
has an unusual structure. It has no TATA box
and the putative initiator element is not the
main TIS selector. Instead, it contains six
27-bp long conserved tandem repeats between
position 70 and position 225 relative
to the major TIS, which both direct efficient
transcription and determine the TIS. Interestingly,
this promoter element works independently
of its orientation and is able to stimulate
transcription from a heterologous promoter.
Finally, the identification of histone acetyltransferases
and histone deacetylases in apicomplexan
parasites suggests that transcription
in these parasites is also regulated by histone
modification. In other eukaryotes, it was shown
that acetylation of lysines in the N-terminus of
core histones affects the nucleosomal structure
and typically is associated with transcription
activation. Conversely, histone deacetylation
generally leads to inactivation or transcriptional
silencing.
Helminths
Our knowledge about class II transcription
of protein coding genes in nematode and
trematode parasites is very limited. However,
helminths are phylogenetically placed between
yeast and vertebrates, and most likely
share the basal RNA pol II machinery characterized
in these organisms. Furthermore,
the relative relatedness of helminths to these
organisms should facilitate the identification
of transcription factors by homology.
Thus far, the CCAAT-binding factor NF-Y in
Schistosoma mansoni and the TBP-related factor
2 of Brugia malayi have been recognized
in this way.
SL RNA AND U snRNA GENE
TRANSCRIPTION IN
NEMATODES AND
TRYPANOSOMATIDS
Trans-splicing of a SL to nuclear pre-mRNA is
an essential maturation step for all or the
majority of mRNAs in trypanosomatid and
nematode parasites, respectively. Since SL
RNA molecules are destroyed in the transsplicing
process, a high rate of SL RNA synthesis
is crucial for the survival of these
organisms. Interference with SL RNA gene
transcription seems to be a promising strategy
to inhibit parasite growth. SL RNAs are small,
non-polyadenylated RNAs resembling U
snRNAs and, in nematodes and trypanosomatids,
are synthesized by RNA pol II. In
higher eukaryotes, U snRNA gene promoters
which recruit RNA pol II consist in general
of a proximal sequence element (PSE)
located around position 55 and a short distal
sequence element (DSE) 170 bp further
upstream (Figure 3.2). The PSE is the core
promoter and binds the multi-subunit basal
transcription factor SNAPc, whereas the DSE
is an enhancer element. Approximately 15 bp
downstream of the U snRNA coding region,
a conserved element, designated the ‘3 box’,
MOLECULAR BIOLOGY