RÉSUMÉ
Full length cDNAs of Cucumber mosaic virus(CMV)CB7 strain,causing necrosis on Nicotiana glutinosa,were obtained by RT-PCR,using viral genomic RNAs as templates.cDNAs of CMV-CB7 genomic RNAs were cloned and sequenced and results indicated that RNA1,2 and 3 was 3 356 nt,3 045 nt and 2 218 nt,respectively(accordingly Accession Number EF216866,DQ785470 and EF216867).Infectious RNA transcripts from cDNA clones of CMV-CB7 were inoculated onto N.glutinosa and the seedlings of host plants displayed necrosis symptom,whist that of CMV-Fny induced typical mosaic symptoms.Through pseudorecombination between CMV-CB7 and CMV-Fny genomic RNAs,the genetic determinant of necrosis phenotype was mapped to RNA2.Chimeric infectious clones consisting of partial sequences of RNA2 derived from CMV-CB7 and CMV-Fny,respectively,were obtained by Overlapping PCR.Pathogenic analysis with those chimeric RNA2 revealed that 2b gene or 3' UTR of CMV-CB7 RNA2 was responsible for the necrotic pathotype.Northern blotting analysis reflected that both necrotic and non-necrotic viruses accumulated to similar levels of genomic RNAs in host plants.Therefore,necrotic phenotype induced on N.glutinosa was not related to the level of accumulation of CMV genomic RNAs.