IDENTIFICATION OF THE COAT PROTEIN GENE OF A SWEET-POTATO SUNKEN VEINCLOSTEROVIRUS ISOLATE FROM KENYA AND EVIDENCE FOR A SEROLOGICAL RELATIONSHIP AMONG GEOGRAPHICALLY DIVERSE CLOSTEROVIRUS ISOLATES FROM SWEET-POTATO

A Kenyan isolate of sweet potato sunken vein virus (SPSVV-Ke), a tenta tive member of the genus Closterovirus, was transmitted to Ipomoea ser osa by the whitefly Bemisia tabaci. Cross-banded filamentous particles about 850 nm in length were detected in infected plants by immunoelec tron microscopy (IEM) with an antiserum to virions of an Israeli isola te of SPSVV (SPSW-Is). Viral double-stranded RNA species of about 10 a nd 9 kbp were extracted from infected I. serosa and used as templates for complementary DNA (cDNA) synthesis. Sequencing of selected cDNA cl ones revealed an open reading frame of 774 nucleotides that encodes a protein with an estimated molecular mass of 29,028 Da. Computer analys is of the deduced amino acid sequence of this protein indicated a dist inct affinity to the coat protein (CP) of lettuce infectious yellows d osterovirus (LIYV) and a lesser similarity to the CPs of beet yellows and citrus tristeza closteroviruses, suggesting that it is the CP of S PSW-Ke. After expression of the CP gene of SPSVV-Ke in Escherichia col i, its identity as the viral CP was confirmed by Western blot analysis with the SPSVV-Is antiserum. This antiserum and a rabbit antiserum ra ised against the bacterially expressed CP of SPSVV-Ke were used in Wes tern blot and IEM experiments for assessing the serological relationsh ips among SPSVV-Ke, SPSVV-Is, and sweet potato virus disease-associate d closterovirus isolates from Nigeria and the United States. Results s howed that SPSVV-Ke is closely related serologically to similar closte rovirus isolates infecting sweet potato in Israel, Nigeria, and the Un ited States but differs from them in reacting weakly with an antiserum to LIYV in IEM and Western blots.