Virus purification. Both YC5 and RC4
isolates were individually propagated in
mustard plants and purified by differential
and cesium sulfate equilibrium centrifuga-
tions. Purified virion preparations had a
UV absorption spectrum and 260:280 ab-
sorption ratio of 1.24, typical of potyvi-
ruses (28). The estimated yields of purified
YC5 and RC4 were 2 and 3 mg per 100 g
of tissue, respectively. The estimated mo-
lecular mass of coat protein in 12% poly-
acrylamide gel was 37 kDa (PAGE, data
not shown), similar to that reported for
other TuMV isolates (10,23).