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observed in adult male mosquitoes. Despite fast
in vitro
self-scission, the
ribozymes found in adult male mosquitoes appear in their intact, uncleaved
forms. However, self-cleaved forms of the ribozyme are readily detected in
both pupal and larval stages of the insect and show lower overall expression
relative to the adult, indicating that both transcription and self-scission
events are regulated
in vivo
.
Unlike expression, which can be regulated on transcriptional or post-
transcriptional levels, regulation of self-scission is likely due to the direct
alteration of ribozyme structure to prevent formation of the catalytic core.
Such ribozyme disruption might be achieved by binding an antisense tran-
script, protein, or small molecule effector, or by forming an alternative sec-
ondary structure with a distal element of the same transcript. Alternative
folding, however, would require formation of a competing structure in a
sex- and development-specific manner that would again likely involve
nucleic acid, protein, or other effectors.
The genomic distribution of the drz-Agam ribozymes is generally not
conserved, as the sequences map to introns and intergenic regions of the
genome. However, BLAT analysis of the drz-Agam-2 sequences placed
them at the 5
0
end of the non-long terminal repeat (non-LTR) ret-
ransposable element (RTE).
3,120
This retrotransposon was first identified
in
Caenorhabditis elegans
, and encodes an apurinic-apyrimidinic (AP) endo-
nuclease and a reverse transcriptase protein.
121
In a typical non-LTR
retrotransposition event, the 5
0
end of the motif is often truncated due to
inefficient or incomplete retrotransposition followed by insertion into the
host genome. Thus, it is somewhat surprising to find the highly complex
drz-Agam-2-2 sequence in its entirety forming the 5
0
terminus of the ele-
ment. That this sequence appears in its full-length, active form suggests that
the ribozyme-catalyzed self-cleavage event is necessary for the viability of
the RTE. Recently, HDV-like ribozymes have been found at the beginning
of additional classes of transposable elements, supporting a model in which
self-scission by the ribozyme is
responsible for
the cotranscriptional
processing of the elements.
4
5.9. R2 ribozymes
The best-understood class of retrotransposons found to be associated with
HDV-like ribozymes is the R2 family.
122
This class of transposons in-
serts site-specifically into the 28S rDNA of most insect species. It
encodes an endonuclease and reverse transcriptase protein for use in the
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