3.5.8
NUCLEOTIDE SEQUENCE ANALYSIS OF AN ATTENUATED STRAIN OF CUCUMBER MOSAIC VIRUS


M SUZUKI 1, T NATSUAKI 1, Y KOSAKA 2, K NAKADA 1 and S OKUDA 1

1 Faculty of Agriculture, Utsunomiya University, Mine-machi, Utsunomiya 321-8505, Japan; 2 Kyoto Biotechnology Center, Kitainayadzuma, Seika, Soraku, Kyoto 619-02, Japan

Background and objectives
Cucumber mosaic cucumovirus (CMV) is economically important to cause serious diseases in many crops in Japan. Cross-protection has been used successfully to control CMV in several crops with attenuated strains. Symptom modulation mediated by CMV satellite RNA has been extensively studied by many laboratories. However, molecular information about attenuated CMV strains which are not mediated by satellite RNA and about the mechanism of cross-protection is limited. An attenuated isolate CM95, not containing satellite RNA, is widely used for cucumber production in Kyoto prefecture. The objective of this study was to sequence the genomic RNA of effective attenuated isolate CM95 [1] and to understand which RNA segment may affect the pathogenicity of CM95 at the molecular level.

Materials and methods
The CM95 without mild satellite RNA was obtained from the III-19 by three serial single-lesion passages. The III-19 was a single-lesion isolate from a plant co-inoculated with two CMV isolates, 36a1 and Fuka4-4. The 36a1 isolate produced mild symptoms on almost all plants tested and a low degree of protection on cucumber plants. In contrast, Fuka4-4 was excellent as a protecting strain but induced severe symptoms on many plants. The cDNA clones from genomic RNAs of three CMV isolates were sequenced.

Results and conclusions
The complete nucleotide sequences of genomic RNA1-3 of three CMV isolates (36a1, Fuka4-4, CM95) were determined. Analysis of the sequences clearly reveals that RNA1 and RNA2 of CM95 have derived from 36a1 and RNA3 of CM95 from Fuka4-4. The proteins encoded by RNA1 and RNA2 of CM95 were different at four amino acid positions from those of 10 virulent CMV strains. These data suggest that RNA1 and RNA2 determine the pathogenicity of CM95 and RNA3 determines the ability of cross-protection on cucumber plants.

References
1. Kosaka Y, Fukunishi T, 1997. Plant Disease 81, 733-738.