Replication of DNA Satellites and Their Role in Viral Pathogenesis

The white-fly borne begomoviruses (family Geminiviridae) have circular single-stranded (css) DNA genome, which is encapsidated as monopartite (DNA-A) or bipartite (DNA-A and DNA-B) in the twinned icosahedrons. During the course of their evolution and to escape host defense machinery, begomoviruses adopt small cssDNA satellites called alpha-, beta-, and deltasatellites. Alphasatellties are found to be associated with begomovirus–betasatellite complexes and encode their own replication-associated protein (Rep), thus capable of autonomous replication. These satellite-like molecules are not well known to serve any critical function for their helper begomovirus except for few reports about attenuation of helper-virus accumulation and/or occasionally suppression of the host defense. Most of the monopartite begomoviruses in the Old World (OW) are found to be associated with betasatellites; however, none of the New World (NW) begomoviruses are known to be associated with betasatellites. Begomoviruses replicate their genome through rolling circle replication (RCR), which requires the virus-encoded Rep to recognize and bind to the iterated sequences (iterons) in the origin of replication (ori) region. Betasatellites lack such iterated sequences; however, they can be transreplicated by a diverse range of begomoviruses, following a similar pattern for replication. Betasatellites play a significant role in viral pathogenesis by interacting with certain host factors, attenuation of disease symptoms, suppression of host defense, and sometimes inter- or intracellular shuttling of begomovirus genome. Likewise, the noncoding molecules deltasatellites depend upon their helper virus for their replication. However, their precise role in viral pathogenesis still needs to be explored.