Biology Reference
In-Depth Information
operons arose relatively recently, since most branches of these phyla do not have
these features (
Douris et al., 2010
).
D. 3
0
End Formation
1. Canonical 3
0
Processing Factors in Worms
In C. elegans, orthologs of all members of each of the 3
0
end formation complexes
can be found, although some differences in their functionality have been proposed.
For instance, it has been observed that the polyadenylation signal in C. elegans is
more variable than seen in other organisms. Only around 50%of all genes contain the
canonical AAUAAA sequence. Instead, many different variations of this sequence
are employed, including AAUGAA, UAUAAA, GAUAAA, CAUAAA, UAUGAA,
and AUUAAA (
Hajarnavis et al., 2004
). Additionally, roughly 7% of genes have no
recognizable signal related to AAUAAA at all (
Blumenthal and Steward, 1997;
Salisbury et al., 2006
). This sequence variability indicates that CPSF-160 may
recognize this signal differently in C. elegans than it does in higher metazoans, even
though the protein sequence is reasonably well conserved. Furthermore, analysis of
3
0
cleavage sites in C. elegans ESTs has shown that RNA cleavage typically occurs
20 nucleotides downstream of the polyadenylation signal (
Hajarnavis et al., 2004
).
Studies examining the mechanisms behind these differences have not been
performed.
Similar to CPSF-160, the CstF-64 protein seems to have also become specialized,
although its exact role remains unclear. While the mammalian CstF-64 protein
contains 597 amino acids, the C. elegans ortholog contains only 336 amino acids.
Conservation between the two proteins extends only through the N-terminal region
(approximately the initial 200 amino acids.) The entire C-terminal section of the
C. elegans CstF-64 aligns very poorly to its mammalian counterpart (
Salisbury et al.,
2006
), an indication that this section of the protein may have evolved a specialized
function in nematodes. Furthermore, bioinformatic analysis has indicated that the
CstF-64 binding site on the mRNA is not the canonical GU-rich sequence
25 nt
downstream from the cleavage site, as is found in mammals, but rather a U-rich
region just 3
0
to the site of cleavage (
Graber et al., 2007
).
2. Additional Proteins Implicated in 3
0
Processing
Recently, several additional proteins have been implicated in 3
0
end formation in
C. elegans (
Cui et al., 2008
). The genes encoding these proteins were discovered in a
screen for suppressors of the Muv phenotype of an unusual deletion/insertion allele
in the lin-15 operon that revealed several 3
0
end formation and transcription termi-
nation proteins. The screen was conducted in a C. elegans strain in which a genomic
fragment containing the 3
0
end and downstream region of the nonoperon gene
H18N23.2 had been transposed into the third exon of the lin-15B gene. Since lin-
15B is in an operon, upstream of lin-15A, insertion of this site not only inactivates
Search WWH ::
Custom Search