Abstract
The hepadnavirus encapsidation signal, epsilon (ε), is an RNA structure located at the 5′ end of the viral pregenomic RNA. It is essential for viral replication and functions in polymerase protein binding and priming. This structure could also have potential regulatory roles in controlling the expression of viral replicative proteins. In addition to its structure, the primary sequence of this RNA element has crucial functional roles in the viral lifecycle. Although the ε elements in hepadnaviruses share common critical functions, there are some significant differences in mammalian and avian hepadnaviruses, which include both sequence and structural variations.
Here we present several covariance models for ε elements from the Hepadnaviridae. The model building included experimentally determined data from previous studies using chemical probing and NMR analysis. These models have sufficient similarity to comprise a clan. The clan has in common a highly conserved overall structure consisting of a lower-stem, bulge, upper-stem and apical-loop.
The models differ in functionally critical regions-notably the two types of avian ε elements have a tetra-loop (UGUU) including a non-canonical UU base pair, while the hepatitis B virus (HBV) epsilon has a tri-loop (UGU). The avian epsilon elements have a less stable dynamic structure in the upper stem. Comparisons between these models and all other Rfam models, and searches of genomes, showed these structures are specific to the Hepadnaviridae. Two family models and the clan are available from the Rfam database.