The novel tertiary structure in delta RNA may function as a ribozyme control element

The viroid-like domain making up the lefthand end of the delta hepatitis genome (Branch et al., 1989) has structural elements whose interactions may be essential for replication. This portion of the genome is highly conserved in primary sequence and contains two well-defined types of structural features: sites capable of self-cleavage, and those forming a UV-sensitive element of local tertiary structure which is very stable and contains non-Watson:Crick bonds that may form a distinctive surface for binding specific proteins. The proximity of the tertiary structure to the genomic self-cleavage site suggests that the photoreactive element may regulate the ribozyme. This element's stability would limit "breathing" in this region of the circular genome, maintaining the ribozyme in the "off" conformation; its tight structure could be relieved by protein binding at the time of replication. We previously mapped a novel local tertiary structure to a highly conserved portion of the viroid genome (Branch et al., 1985). The many additional properties shared by viroids, related infectious circular RNAs of plants, and the delta agent are discussed in a recent article (Branch et al., in press-a).