trans

44 RNA PROCESSING
stage. However, since the abundance of gRNAs
does not correlate well with the level of edited
mRNAs, regulatory mechanisms other than
simple transcript abundance are likely to affect
editing efficiency. In addition, editing could
possibly be linked to other processes, including
RNA synthesis, maturation (polyadenylation,
alternative processing), or message stability.
And because it is currently unknown whether
partially edited mRNAs are translated, the
potential also exists for the production of different
protein isoforms during development.
SUMMARY
There are a number of notable features of
kRNA editing that make this form of gene
expression unique. These include the findings
that (i) some (pan-edited) mRNAs require many
separate genetic segments to assemble a functional
mRNA product, (ii) non-Watson–Crick
(G-U) pairs are involved in the production of
mRNA transcripts, and (iii) only part of the
mRNA is copied directly from the DNA, the
rest is templated by gRNA transcripts.
Despite significant differences in mechanism,
there are a number of interesting parallels
between kRNA editing and trans-splicing.
First, each entails the expression and assembly
of genetic information encoded at multiple
locations within the genome. Secondly, in
each case the ultimate end-product of the
‘gene’ is assembled at the RNA level using multiple
substrates. Finally, each of these processes
is apparently catalyzed by large ribonucleoprotein
complexes potentially requiring de novo
assembly at each splice site or editing block.
The existence of these alternative modes of
gene expression within different cellular compartments
of a non-traditional ‘model’ organism
emphasizes the importance of exploring
all evolutionary niches as we seek to elucidate
the underlying mechanisms of gene expression
in all of its diverse forms.
Trans-splicing
FURTHER READING
Agabian, N. (1990). Trans-splicing of nuclear premRNAs.
Cell 61, 1157–1160.
Blumenthal, T. and Steward, K. (1997). RNA processing
and gene structure. In: Riddle, D.L.,
Blumenthal, T., Meyer, B.J. and Priess, J.R. (eds).
C. elegans II, Cold Spring Harbor: Cold Spring
Harbor Laboratory Press, pp. 117–146.
Bonen, L. (1993). Trans-splicing of pre-mRNA in
plants, animals, and protists. FASEB J. 7, 40–46.
Bruzik, J.P., van Doren, K., Hirsh, D. and Steitz, J.A.
(1988). Trans-splicing involves a novel form of
small ribonucleoprotein particles. Nature 335,
559–562.
Davis, R.E. (1996). Spliced leader RNA trans-splicing
in metazoa. Parasitol. Today 12, 33–40.
Denker, J.A., Maroney, P.A., Yu, Y.-T., Kanost, R.A.
and Nilsen, T.W. (1996). Multiple requirements
for nematode spliced leader RNP function in
trans-splicing. RNA 2, 746–755.
Denker, J.A., Zuckerman, D.M., Maroney, P.A. and
Nilsen, T.W. (2002). New components of the
spliced leader RNP required for nematode transsplicing.
Nature 417, 667–670.
Ferguson, K., Heid, P. and Rothman, J. (1996). The
SL1 trans-spliced leader RNA performs an
essential embryonic function in Caenorhabditis
elegans that can also be supplied by SL2 RNA.
Genes Dev. 10, 1543–1556.
Hannon, G.J., Maroney, P.A., Denker, J.A. and
Nilsen, T.W. (1990). Trans-splicing of nematode
pre-messenger RNA in vitro. Cell 61, 1247–1255.
Krause, M. and Hirsh, D. (1987). A trans-spliced
leader sequence on actin mRNA in C. elegans.
Cell 49, 753–761.
Murphy, W.J., Watkins, K.P. and Agabian, N. (1986).
Identification of a novel Y branch structure as an
intermediate in trypanosome mRNA processing:
evidence for trans-splicing. Cell 47, 517–525.
Nilsen, T.W. (1994). RNA–RNA interactions in the
spliceosome: Unraveling the ties that bind. Cell
78, 1–4.
MOLECULAR BIOLOGY