Replication and Pathogenesis of RNA Viruses 
is the hepatitis delta virus (HDV) , a human hepati- 
tis virus commonly associated with a severe form 
of the disease. HDV, by itself, does not infect hu- 
mans, because it is defective and requires another 
viral agent, hepatitis B virus (HBV), to supply an 
essential envelope protein to infect liver cells. 
HDV has been shown to cause epidemics of fulmi- 
nant hepatitis in many parts of the world. In the 
-United States, it is prevalent among intravenous 
drug abusers. 
The virus contains a circular, single-stranded 
RNA genome of only 1,700 nucleotides. It is the 
only animal virus with a circular RNA. This ge- 
nome structure is reminiscent of a group of plant 
pathogens, viroids or virusoids, that cause a vari- 
ety of plant diseases. Indeed, the similarity be- 
tween HDV RNA and plant viroid RNAs goes 
beyond their circular RNA structure. There are 
several structural and biochemical features that 
suggest a close evolutionary relationship be- 
tween HDV and plant viroid RNAs. Both RNA 
groups contain a "ribozyme" activity, in which 
the RNA serves as an enzyme that cleaves and li- 
gates the RNA itself. Thus HDV RNA stands at a 
peculiar place in the evolutionary ladder: it may 
have been derived from a plant pathogen by re- 
combination with a gene that gave it the ability to 
infect human cells and cause diseases. Our labora- 
tory is studying the properties of this ribozyme 
activity. We have determined its structural and 
sequence requirements, which are distinct from 
those of other known ribozymes. Furthermore, 
we have begun to examine whether ribozyme ac- 
tivity is the same in infected cells as in the test 
tube. The data obtained thus far suggest the inter- 
esting possibility that some cellular factors could 
participate in the ribozyme activity. We are pur- 
suing these factors. 
One important difference between HDV RNA 
and plant viroid RNAs is the ability of the former 
to synthesize a protein, hepatitis delta antigen 
(HDAg), the HDV signature protein. HDAg is re- 
quired for RNA synthesis. We have been studying 
this protein's properties and functions and have 
shown that HDAg is a phosphoprotein that resides 
in the nuclei of the cells. It can interact with itself 
to form a protein complex, and it can also inter- 
act with HDV RNA in a specific way. We have 
shown that HDAg utilizes a set of novel sequence 
motifs to allow itself to bind to RNA. 
What is the role of this protein, HDAg, in HDV 
RNA synthesis? — an unusual synthesis because 
HDV RNA is so small (1,700 nucleotides) that it 
lacks the capacity to provide its own synthesizing 
enzymes. Therefore, HDV most likely borrows 
cellular enzymes to do the job, which is unchar- 
acteristic of RNA viruses. Most RNA viruses have 
to make their own enzymes, since normal cells do 
not appear to have this type. We are studying how 
HDV modifies the cellular enzymes to perform 
this rather atypical RNA synthesis and how HDV 
RNA synthesis initiates this. It appears that HDAg 
participates in these processes. 
HDV thus provides a perspective on viral strate- 
gies from the small end of the RNA spectrum. Our 
laboratory is studying one of the largest RNA vi- 
ruses (coronavirus) and the smallest (HDV), 
which utilize different principles for viral repli- 
cation. Our studies not only offer insights into 
how these viruses cause diseases but also into 
fundamental mechanisms of RNA synthesis and 
RNA evolution. 
240 
