Department of Virology, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel

PVYNTN was distinguished from PVYN on the basis of di}erences in electrophoretic mobility of their cor! responding RNA transcripts[ Transcripts were copied from polymerase chain reaction products of two PVY strains to which the T6 promoter was added[ Single! strand and double!strand transcript heteroduplexes of the NTN and N strains exhibited di}erent rate of migration in gel which enabled strain di}erentiation[ This procedure obviates the need for prior information on sequence divergence "e[g[ restriction sites\ etc[# among the virus isolates[