trans

52 TRANSCRIPTION
FIGURE 3.1 Class I promoters of Saccharomyces
cerevisiae, Trypanosoma brucei, and Leishmania
donovani. Schematic drawing to scale of rRNA gene
(rDNA) promoters, and procyclin (PRO) and variant
surface glycoprotein (VSG) gene expression site promoters.
Promoter domains, as identified by linkerscanner
or block substitution analyses (see text), are
indicated by rectangles; they are numbered according
to the yeast nomenclature. Promoter domains important
for stable binding of trans-activating factors in
transcription competition experiments are drawn in
black. The two elements of the T. brucei rDNA promoter
domain IV (striped rectangles) share sequence
homologies with SL RNA gene promoter elements, are
orientated in the opposite direction to that of the SL
RNA gene (arrow) and, in vitro, bind a trans-acting
factor essential for SL RNA gene transcription. The
region of the putative domain III in the T. brucei rDNA
promoter has not been analyzed. Domain IV of the
S. cerevisiae promoter is the Reb1p binding domain
(Reb1) which is involved in chromatin remodeling.
For the T. brucei and L. donovani promoters, the
drawings show minimal promoter regions required
for full transcriptional activity. Positions of the
5 ends are relative to the TISs which are indicated
by flags.
215 which, by binding the protein Reb1, is
involved in chromatin remodeling and is
important for transcription in vivo within the
rDNA repeat array.
In comparison, the two procyclin ES promoters
of T. brucei are nearly identical in
sequence and are similarly structured to the
yeast rDNA promoter (Figure 3.1). According
to detailed mutational analyses in vivo and
in vitro, the region between positions 246 and
7 is sufficient for full transcriptional activity
and contains four distinct promoter domains.
Domain I (40/7) is absolutely essential for
transcription and presumably represents the
core promoter. Alteration of domain II (72/
57) reduced transcription efficiency dramatically
but did not abolish it, and mutation of
domain III (143/90) resulted in a moderate
drop in transcriptional activity. Furthermore,
changing the distance between domains I,
II, and III strongly reduced transcription
efficiency, demonstrating that the positional
arrangement of these promoter elements is of
crucial importance. In all studies, the results
for domains I to III were in close agreement. In
contrast, the most distal part of the promoter
containing domain IV (222/207) proved to
be important in vivo but not in vitro. Hence,
size and location of the T. brucei procyclin ES
promoter domains are very similar to those of
the yeast rDNA promoter. Moreover, in vitro
competition of procyclin ES promoter transcription
revealed that, as in the yeast rDNA
promoter, domain III in cooperation with
domain II is essential for stable binding of
trans-activating factors whereas domain I is
not. The procyclin ES promoter domain IV is
located at approximately the same position as
the Reb1 binding domain of the yeast rDNA
promoter (Figure 3.1) and, since it exerts its
effect only in vivo, it may function in chromatin
remodeling in the same way as its yeast
counterpart.
MOLECULAR BIOLOGY