Okra mosaic virus (OkMV) is a tymovirus infecting members of the family Malvaceae. Early infections in okra (Abelmoschus esculentus) lead to yield losses of 12-19.5%. Besides intensive biological characterizations of OkMV only minor molecular data were available. Therefore, we determined the complete nucleotide sequence of a Nigerian isolate of OkMV. The complete genomic RNA (gRNA) comprises 6,223 nt and its genome organization showed three major ORFs coding for a putative movement protein (MP) of M r 73.1 kDa, a large replication-associated protein (RP) of M r 202.4 kDa and a coat protein (CP) of M r 19.6 kDa. Prediction of secondary RNA structures showed three hairpin structures with internal loops in the 5'-untranslated region (UTR) and a 3'-terminal tRNA-like structure (TLS) which comprises the anticodon for valine, typical for a member of the genus Tymovirus. Phylogenetic comparisons based on the RP, MP and CP amino acid sequences showed the close relationship of OkMV not only to other completely sequenced tymoviruses like Kennedya yellow mosaic virus (KYMV), Turnip yellow mosaic virus (TYMV) and Erysimum latent virus (ErLV), but also to Calopogonium yellow vein virus (CalYVV), Clitoria yellow vein virus (CYVV) and Desmodium yellow mottle virus (DYMoV). This is the first report of a complete OkMV genome sequence from one of the various OkMV isolates originating from West Africa described so far. Additionally, the experimental host range of OkMV including several Nicotiana species was determined.

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Expression vectors were constructed from 35S promoter-containing full-length cDNA clones of Zygocactus virus X (ZVX). The expression of foreign genes was driven by the ZVX coat protein (cp) subgenomic promoter. It was successful only when the variable region downstream of the conserved putative promoter region GSTTAAGTT(X 12-13 )GAA was retained. Most of the ZVX cp gene, except for a short 39 part, was replaced by the corresponding sequence of the related Schlumbergera virus X (SVX) and its cp subgenomic promoter to enable encapsidation of the transcribed RNA by an SVX/ZVX hybrid cp. Vector-expressed cp of Beet necrotic yellow vein virus (BNYVV) assembled in Chenopodium quinoa, Tetragonia expansa and Beta vulgaris leaves into particles resembling true BNYVV particles. The virus produced from these constructs retained its ability to express BNYVV cp in local infections during successive passages on C. quinoa. This ability was lost, however, in the rarely occurring systemic infections.Full-length cDNA clones of plant viruses can be modified in such a way that they allow the expression of inserted foreign genes in plants. Such plant virus-based expression vectors are powerful tools for various applications (Awram et al., 2002;Pogue et al., 2002;Porta & Lomonossoff, 2002;Gleba et al., 2004). Diagnostic or therapeutic proteins or enzymesif need be optimized for specific applications by PCRintroduced mutations -can be produced in plants in a cheaper and safer way than in other eukaryotic systems. Genome portions of plant viruses can be checked for their functions (Gilardi et al., 1998) and their influence on virus multiplication (Culver, 1996) without the need for producing transgenic plants. Functions of plant genes, including resistance genes, may be assessed, because vector-expressed gene portions are multiplied via a double-stranded RNA step, making the genes susceptible to virus-induced gene silencing (Lu et al., 2003;Brigneti et al., 2004). Most plant virus expression vectors used so far are based on Tobacco mosaic virus or Potato virus X, which mainly infect solanaceous hosts. For studying various aspects of the Beet necrotic yellow vein virus (BNYVV)-induced rhizomania disease, we were interested in expression vectors that would be able to initiate infections in experimental and natural hosts of BNYVV, such as Chenopodium quinoa, Tetragonia expansa and Beta vulgaris. Viruses on which such vectors would be based should have a number of properties favouring the practical application of the anticipated expression systems: they should cause mild systemic infections in the above-mentioned hosts of BNYVV; for safety reasons, they should not be dangerous pathogens for any important crops; they should have elongated particles to avoid packaging problems due to enlarged RNAs; for convenient handling, they should preferably have monopartite genomes; and related viruses should be available for exchanging promoters and other genome elements. Potexvirus isolates from various genera in the Cactaceae, originally grouped toge...

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