It should be clear from the above that all three of the major HDV RNAs arise by post-transcriptional RNA processing. This means that for each, there are precursor RNAs, of relatively larger size, that act as the substrates for such processing. There is much more yet to be revealed about how such important precursor RNA species are initiated and how they are processed.

1. As discussed in Sect. 3.4, an unsubstantiated aspect of the rolling-circle model of replication is whether the RNA species that go ontobe processed into antigenomic RNA circles are initiated from the same location as those which become mRNA species.

2. For the nascent genomic RNA transcripts we have as yet no clear data as to where they are initiated. One report has suggested, based on RNase protection assays, that there might be a preferred site, near one end of the rod-like RNA, and almost opposite to the site that corresponds to the 5'-end of the mRNA (Beard et al. 1996). This result needs to be independently confirmed.

3. Since the nascent transcripts of both the genomic and antigenomic RNA each contain a ribozyme domain per unit length, a transcript that is greater than unit length can contain two or more ribozyme domains, which will lead to ribozyme processing, to release RNA species that are of exactly unit length, and that can then be converted to unit length RNA circles. There is good evidence that the ribozyme domains are needed for the cleavage events to release the unit length linear RNAs (Macnaughton et al. 1993). There is also a report that the subsequent conversion of these to circles depends upon host factors, probably a host RNA ligase (Reid and Lazinski 2000). It will be important to identify this host ligase and determine how it is redirected to act on HDV RNAs.

4. During HDV replication additional processed RNAs that are relatively less abundant can be detected. Northern analyses can detect molecules that seem to be of twice or even three times unit length. Moreover, these species seem to exist in both circular and linear conformations (Chen et al. 1986). Apparently these species arise via alternative processing of multimeric nascent RNA transcripts.

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