Evidence for a model ancestral viroid.

The generation of a phylogenetic tree of viroids and viroid-like plant satellite RNAs via computer analysis, coupled with several conspicuous biochemical characteristics of the rolling circle replication of these RNAs -including both self-cleavage and self-ligation- leads us to propose the peach latent mosaic viroid (PLMVd) as a current "living fossil" dating from a precellular world. Incorporated within this proposal is a revised mechanism of PLMVd rolling circle replication which requires a minimal protein involvement.