Small Circular Satellite RNAs

The infectivity of small circular satellite RNAs (sc-satRNAs) depends on a helper virus. However, sc-satRNAs resemble viroids in several features, including the small (220!257 nt) and single-stranded genome, the accumulation in the infected host as covalently-closed circular RNAs, the replication through rolling-circle mechanisms based on RNA intermediates only, and the inability to code for proteins, with one exception. In addition, sc-satRNAs are endowed with self-cleaving (and some with self-ligation) activity mediated by ribozymes, another feature shared with some viroids, thus reinforcing the hypothesis that these two groups of subviral infectious RNAs may have a monophyletic origin (Elena et al., 2001). However, a key biological difference exists between sc-satRNAs and viroids regarding the molecular machinery needed for replication, which is supplied by the helper virus and the host in the first case, but only by the host in the second. In addition, in contrast to viroids, sc-satRNAs are encapsidated by the helper virus coat protein. This chapter focuses on the main structural and biological features of sc-satRNAs, pointing out the role of hammerhead and hairpin ribozymes in their replication. Readers looking for additional information on sc-satRNAs may check previous reviews