Genomic characterization of pea enation mosaic virus-2 from the Pacific Northwestern USA.

Abstract

Pea enation mosaic virus (PEMV) infects several legume crops, including chickpea (Cicer arietinum), faba bean (Vicia faba), lentil (Lens culinaris) and pea (Pisum sativum). The virus caused yield losses of food legumes in the Pacific Northwestern US during 1983, 1987 and 1990 [1]. Our recent surveys of pea and alfalfa fields in the states of Washington and Idaho, USA, have shown the prevalence of PEMV on pea. PEMV consists of a large (RNA-1 or PEMV-1) and a small (RNA-2 or PEMV-2) single-stranded positive-sense RNA, which are encapsidated separately into distinct isometric particles [2]. PEMV-2 is one of the seven distinct virus species in the genus Umbravirus. The other species of this genus are Carrot mottle virus (CMoV), Carrot mottle mimic virus (CMoMV), Groundnut rosette virus (GRV), Lettuce speckles mottle virus (LSMV), Tobacco mottle virus (TMoV) and Tobacco bushy top virus (TBTV) [3]. Members of this group lack the coat protein (CP) gene in their genomes and depend on a helper virus for survival [4]. PEMV-2 in association with PEMV-1 (genus Enamovirus), form a symbiotic bipartite virus complex referred to as PEMV. It has been suggested that RNA-1 and RNA-2 are essential for PEMV infection [4, 5]. The RNA-1 (5706 nucleotides) encodes five major ORFs (1-5) that have nucleotide and amino acid sequence similarities with subgroup II luteoviruses [5]. While PEMV-1 provides the necessary encapsidation and vector-transmission abilities for PEMV-2, the latter provides PEMV-1 with long-distance movement and mechanical transmission functions. As part of an ongoing project to determine the genetic diversity of PEMV, we recently characterized the genome of PEMV-1 from the Pacific Northwest [6]. So far, there are only two complete genomic sequences of PEMV-2 in GenBank (NC_003853 and AY714213). To better understand sequence diversity of PEMV-2, the genome structure and organization of two PEMV isolates, one from Idaho (PEMV-2-ID) and one from Washington (PEMV-2-WA), were determined in this study. The PEMV-2 genome (*4.2 kbp), like that of other umbraviruses, predominantly consists of four ORFs (1-4), which perform diverse functions [4, 7, 8]. RNA-2 lacks a polyadenylation signal at its 30 end and contains a large 50 genome-linked protein [9]. ORF-1, at the 50 end of the virus genome, initiates after a short, 20-nt non-coding region (NCR) and encodes a putative 33-kDa protein of unknown function. ORF-2 overlaps with ORF-1 at its 30 end and potentially encodes a protein of 65 kDa through a frameshift mechanism. ORF-2 contains sequence motifs characteristic of a viral RNA-dependent RNA-polymerase (RdRp). The presence of a polymerase cassette in RNA-2 also reveals its independent replicative capabilities. Also, due to the presence of an octanucleotide frameshift signal ‘‘GGATTTTT’’ immediately upstream of the stop codon of ORF-1, ORF-1, along with ORF-2, is expressed by a -1 frameshift mechanism as a minor translation product of 97 kDa [4]. ORFs 3 and 4 in the genome occur after a nonThe sequences described here were deposited in the GenBank database with the following accession numbers: JF713435 for PEMV2-WA and JF713436 for PEMV-2-ID.