The entire sequences of two related virusoids, the RNA 2 from lucern transient streak virus (LTSV) and that from Solanum nodiflorum mottle virus (SNMV), and the E coli M1 RNA were found to be homologous to the distal 380-base region of the E. coli rrnD operon (dis-rrnD) consisting of two 5S rRNA genes, tRNA(Thr1) gene, and short spacers. Virusoids and M1 RNA were thus concluded to have evolved from primitive dis-rrnD homologue(s), negating the hypothesis that virusoids might be direct descendants of an early self-replicating RNA molecule before the emergence of the first genetic apparatus. Substantial evidence was also found which supports the previously proposed hypothesis that tRNA first evolved by duplication of an ancestral half-sized RNA ("semi-tRNA") homologus to both the 5'- and 3'-halves of tRNAs. Semi-tRNA is a plausible candidate for an early possibly self-replicating RNA.
109 - 120