Identification of the coat protein gene of a sweet potato sunken vein closterovirus 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 tentative member of the genus Closterovirus, was transmitted to Ipomoea serosa by the whitefly Bemisia tabaci. Cross-banded filamentous particles about 850 nm in length were detected in infected plants by immunoelectron microscopy (IEM) with an antiserum to virions of an Israeli isolate of SPSVV (SPSW-Is). Viral double-stranded RNA species of about 10 and 9 kbp were extracted from infected I. serosa and used as templates for complementary DNA (cDNA) synthesis. Sequencing of selected cDNA clones revealed an open reading frame of 774 nucleotides that encodes a protein with an estimated molecular mass of 29,028 Da. Computer analysis of the deduced amino acid sequence of this protein indicated a distinct affinity to the coat protein (CP) of lettuce infectious yellows dosterovirus (LIYV) and a lesser similarity to the CPs of beet yellows and citrus tristeza closteroviruses, suggesting that it is the CP of SPSW-Ke. After expression of the CP gene of SPSVV-Ke in Escherichia coli, its identity as the viral CP was confirmed by Western blot analysis with the SPSVV-Is antiserum. This antiserum and a rabbit antiserum raised against the bacterially expressed CP of SPSVV-Ke were used in Western blot and IEM experiments for assessing the serological relationships among SPSVV-Ke, SPSVV-Is, and sweet potato virus disease-associated closterovirus isolates from Nigeria and the United States. Results showed that SPSVV-Ke is closely related serologically to similar closterovirus isolates infecting sweet potato in Israel, Nigeria, and the United States but differs from them in reacting weakly with an antiserum to LIYV in IEM and Western blots.