1.13.3
cDNA CLONING AND NUCLEOTIDE SEQUENCES OF THE COAT PROTEIN GENES OF TWO STRAINS (YS AND YP) OF SOYBEAN DWARF LUTEOVIRUS

S KANEMATSU1, S HIDAKA1, K HONDA1, Y MIKOSHIBA2 and K ISHIGURO1

1Tohoku National Agricultural Experiment Station, Morioka, Iwate, Japan; 2National Agriculture Research Center, Tsukuba, Ibaraki, Japan

Background and objectives
Soybean dwarf virus (SDV) is, from a viewpoint of symptoms, divided into two groups, namely the yellowing (Y) strain and the dwarfing (D) strain. The Y strain is further divided into two groups which are characterized by their aphid transmissibility. One (YS) is specifically transmitted by Aulacorthum solani and the other (YP) is transmitted experimentally by Acyrthosiphon pisum. The complete genome of the Tasmanian isolate (Tas-1) transmitted by A. solani has been sequenced [1], and the coat protein (CP) gene of the YS strain (YSh) isolated in Hokkaido and maintained in the USA was also done [2]. For the purpose of clarifying the genetic differences between YS and YP, or YP and bean leaf roll luteovirus (BLRV), which is transmitted by A. pisum, we sequenced the CP genes of the Morioka (northern Japan) isolates of YS and YP (YSm and YPm).

Materials and methods
Primers were designed so as to amplify the entire CP gene of the SDV genome by RT-PCR [1]. The amplified (ca 700 bp) fragments were blunt-ended, cloned into the SmaI site of pGEM-3Z and sequenced using the DyeDeoxy Terminator Cycle Sequencing method (Perkin Elmer).

Results and conclusions
Both of the CP genes were composed of 600 nt, and the 21-K genes, which are overlapped in another frame in the same region, were composed of 567 nt. These sizes corresponded to those of Tas-1 and YSh isolates. The homology percentages of the CP genes and the 21-K genes between YSm, YPm, Tas-1 and YSh were 96.7-98.5 and 96.5-98.4%, respectively. The deduced amino acid (AA) sequences of CP and 21-K shared 96.0-98.5 and 93.1-97.4% similarity among four SDV isolates. A comparison of their CPAA sequences with those of other luteoviruses revealed that the homology percentage between SDV-YSm and BLRV was 63.9%, and that between SDV-YPm and BLRV was 62.8%. These data led us to conclude that SDV-YP was a strain of SDV and distinctly different from BLRV.

Recently we demonstrated that SDV-YP was transmitted by Nearctaphis bakeri in the field, and N. bakeri also transmitted other undescribed virus strains which had the same antigen-recognition site as YP and caused extreme dwarfing in soybeans (unpublished data). We will further investigate the relationships between SDV strains and their vectors in the near future.

References
1. Rathjen JP, Karageorgos LE, Habili N et al., 1994. Virology 198, 671-679.
2. Smith OP, Damsteegt VD, Harris KF, Vonder Haar R, 1993. Archives of Virology 133, 223-231.