Molecular characterization of two evolutionarily distinct endornaviruses co-infecting common bean (Phaseolus vulgaris)

Abstract

Two high-molecular-mass dsRNAs of approximately 14 and 15 kbp were isolated from the common bean (Phaseolus vulgaris) cultivar Black Turtle Soup. These dsRNAs did not appear to cause obvious disease symptoms, and were transmitted through seeds at nearly 100% efficiency. Sequence information indicates that they are the genomes of distinct endornavirus species, for which the names Phaseolus vulgaris endornavirus 1 (PvEV-1) and Phaseolus vulgaris endornavirus 2 (PvEV-2) are proposed. The PvEV-1 genome consists of 13,908 bp and potentially encodes a single polyprotein of 4496 aa, while that of PvEV-2 consists of 14 820 bp and potentially encodes a single ORF of 4851 aa. PvEV-1 is more similar to Oryza sativa endornavirus, while PvEV-2 is more similar to bell pepper endornavirus. Both viruses have a site-specific nick near the 5' region of the coding strand, which is a common property of the endornaviruses. Their polyproteins contain domains of RNA helicase, UDP-glycosyltransferase and RNA-dependent RNA polymerase, which are conserved in other endornaviruses. However, a viral methyltransferase domain was found in the N-terminal region of PvEV-2, but was absent in PvEV-1. Results of cell-fractionation studies suggested that their subcellular localizations were different. Most endornavirus-infected bean cultivars tested were co-infected with both viruses.