Virioids and satellite viruses

p 4 VIROIDS AND SATELLITE VIRUSES Viroids are small single-stranded circular RNAs that infect plants (Flores et al , 2004 . These subviral pathogens are characterized by complementary base pairs and long secondary structures . Their genome is approximately 400 nucleotidees in length . RNA viriods do not result in any protein hence they are called unencapsulated , but they multiply by themselves without the help of any other viruses . Their replication takes place in the infected cells . Satellite viruses are typically similar to viroids but these RNA viruses are situated within the protein coat or capsids

of specific helper viruses . Such parasitic locations of satellite viruses are necessary for them to replicate . A prime example of a satellite virus is the human hepatitis ae virus (HDV ) which is dependent of the hepatitis B virus for the packaging and spreading of the HDV to other organisms (Taylor , 2003 . The RNA of the human hepatitis ae virus is composed of a viroidlike domain , which is a segment that shows remarkable similarities to viroids and satellite viruses . Viroids and satellite viruses replicated through the employment of oligomeric RNA intermediates that undergo the rolling circle scheme of DNA replication . The genome of the HDV is composed of a tiny single-stranded circular negative sense RNA of approximately 1 ,700 nucleotides in length . Within the genome , self-cleaving motifs such as the delta ribozymes and an open reading frame are located and these code for two viral proteins There are several hypotheses that have been suggested to elucidate the evolution and origin of viroids . One of the theories suggested that viroids probably came from retroviruses or transposable elements that have lost their interior sequences . Such scenario would translate into the concept that viroids are introns or non-coding gene segments that escaped from another virus . Another theories on the evolution and origin of viroids describes that they are an advanced for or antenna or signal RNAs which eukaryotic cells considered as unreactive The discovery of viroids and satellite viruses thus supports the concept that RNA existed first before DNA . The feature of certain RNAs having catalytic or reactive domain also supports the notion that RNA preceded DNA in terms of evolution . Several investigations have also described primitive types of RNA molecules that showed catalytic properties which are considered as evolutionary relics of the earlier existing RNA world . In addition , the idea that a common ancestor to viroids and satellite viruses must have existed is also unquestionable due to the fact that there are significant similarities between their sequences and catalytic domains . Such monophyletic origin of viroids and viroidlike satellite RNAs are thus a very interesting for research and investigation Viroids are categorized into two families according to the presence or absence of conserved regions , hammerhead ribozymes and their location in within the cell . There are two common areas wherein viroids are typically situated . They are either found in the nucleus or in the chloroplast . Satellite viruses , on the other hand , are further categorized into three groups . The large satellite viruses generate proteins...