Purification, characterization and serological properties of two virus isolates associated with the maize stripe disease in Mauritius

Abstract

Two virus isolates associated respectively with the maize stripe and the maize chlorotic stripe syndromes in Mauritius, denoted MSM and MCS, have been purified and characterized. A single nucleoprotein band of buoyant density 1.289 g/ml was recovered from both MSM and MCS extracts after isopycnic centrifugation in CsC1 gradients. Rate zonal centrifugation in 10–40% sucrose gradients of extracts of leaves infected by each isolate resulted in five light-scattering zones. The highest absorbency at 254 nm was associated with the third fraction from the top. Analysis of purified proteins by SDS-PAGE revealed a single capsid protein of Mr 34000 in preparations of each isolate. Large quantities of a non-capsid protein ofMr 23000 were extracted from both MSM- and MCS-infected material with a concentration of 1 75 and 2.35 mg/g of infected tissue, respectively. No such bands were detected in preparations from healthy leaves or leaves infected with maize streak virus (MSV). Both the 34000 and 23 000 proteins could be recovered from partially purified virus extracts. The property of the non-capsid protein to solubilize above and crystallize below pH 60 was used as the basis for a rapid diagnostic test. Electron microscopy of purified preparations of both isolates showed filamentous particles less than 5 nm in diameter and of different lengths. Several particle configurations, including circular, coiled and unfolded structures, were present, and were typical of a tenuivirus. Antisera raised in rabbits against the two nucleoproteins, each had titres of 1/32 both in homologous and reciprocal Ouchterlony double-diffusion tests. Sap extracts of both MSM- and MCS-infected plants gave strong reactions with antisera to maize stripe virus (MSpV) from Reunion, Kenya and the USA. The most sensitive diagnosis was achieved using F(ab′)2 ELISA. Both MSM and MCS should be considered isolates of maize stripe virus (MSpV). Although MSM and MCS induce different symptoms, they were indistinguishable in our tests; we propose that they are the same and that symptom differences arise as a result of late and early infection, respectively.