Properties of a host range and coat protein variant of broad bean true mosaic comovirus from Vicia faba

Abstract

Summary Unlike other described isolates of broad bean true mosaic comovirus (BBTMV), a variant, code name SB, infected some non‐leguminous plant species and, in N. benthamiana, induced systemic mottling and puckering of the leaves. However, like other described BBTMV isolates, purified SB particle preparations contained isometric particles c. 28 nm in diameter that sedimented as two nucleoprotein components with S20, w values of 90S and 109S; some preparations occasionally contained a component of c. 50S. Virus particles contained two ssRNA species which, when denatured in glyoxal, had estimated MT values of 2.1 × 106 and 1.3 × 106 and co‐electrophoresed with cowpea mosaic virus RNA‐1 and RNA‐2 respectively. Isolate SB was serologically indistinguishable from British and German isolates of BBTMV. However, SB virus particles contained a major polypeptide (L) of Mr between c. 31 000 and up to three minor ones (S) or Mr between c. 20 000 and 24 000. This contrasts with protein preparations from other BBTMV isolates that typically contain only two polypeptides of Mr c. 37 000 (L) and 21 000 (S). Following isopycnic centrifugation in CsCl, SB particles purified from pea separated into two major components with densities of 1.39 and 1.44 g cm‐3 and a minor component of estimated density 1.43 g cm‐3. In Cs2SO4, virus preparations separated into three major components with densities of 1.30, 1.32 and 1.36 g cm‐3 and a minor one of density 1.27 g cm‐3. In CsCl isopycnic gradients, SB particles purified from TV. benthamiana separated into two components with densities of 1.38 and 1.43 g cm‐3. During immuno‐electrophoresis in agarose gels, freshly prepared virus and preparations stored for up to 4 days at 4°C contained a single component that migrated rapidly to the anode, whereas similar preparations of an English isolate of BBTMV migrated as a single component that moved only slowly toward the anode but which, within 48 h, contained an additional component with a migration rate similar to that of isolate SB. Isolate SB is therefore a host range variant of BBTMV which, in comparison with previously described isolates of BBTMV, has an increased negative charge of its particles prior to any appreciable degradation of its S protein, and S protein that is degraded less rapidly. These features probably account for the anomalies observed in isopycnic centrifugation.