1.11.44
SEQUENCES OF THE CIRCULAR ssDNA COMPONENTS OF MILKVETCH DWARF VIRUS IN JAPAN

Y SANO1, M WADA2, Y HASHIMOTO 1, T MATSUMOTO1 and M KOJIMA2

1Department of Applied Biology, Kyoto Institute of Technology, Matsugasaki, Kyoto 606-8585; 2Lab. Plant Pathology, Faculty of Agriculture, Niigata University, Ikarashi 2-8050, Niigata 950-21, Japan

Background and objectives
Milkvetch dwarf virus ( MDV) is a small isometric virus with a circular ssDNA genome [1]. It is transmitted by Aphis craccivora in a persistent manner, and causes yellow dwarf disease on several legume crops in Japan. MDV has structural similarities with other ssDNA plant viruses including subterranean clover stunt virus (SCSV), faba bean necrotic yellows virus (FBNYV), banana bunchy top virus (BBTV) and coconut foliar decay virus (CFDV). To obtain basic information on the ssDNA genome of MDV, and to clarify the relationship with other ssDNA plant viruses, we conducted cloning and sequence analysis of MDV.

Results and conclusions
Ten circular DNA components (C1 to C10) of the MDV genome were identified and sequenced. The sizes of the ten components ranged from 977 to 1022 nt. Each DNA component contained a single open reading frame (ORF) in the virus sense, and had a conserved nonanucleotide sequence (C/TAGTATTAC) in the putative stem-loop structure in the non-coding region. Notably, the four MDV components 1, 2, 3 and 10 encoded the related but distinct 33-kDa replication-initiator proteins (Rep) The major ORFs of six other components, 4, 5, 6, 7, 8 and 9, encoded the putative proteins consisting of 112 to 172 aa with calculated molecular masses ranging from 12.6 to 19.6 kDa. Comparison of the sequences together with those of other ssDNA plant viruses revealed that MDV is closely related to faba bean necrotic yellows virus (FBNYV) and to subterranean clover stunt virus (SCSV). Six putative MDV genome products, including one Rep, the capsid protein and four other non-structural proteins, showed high degrees (70-90%) of aa identity with the corresponding FBNYV proteins, whereas two other Rep proteins encoded by MDV-C 2 and C3 were 82 and 73% identical to that encoded by SCSV-C2 and C6, respectively. The comparison also revealed extensive aa sequence similarity (21-46%) between the corresponding gene products of MDV and banana bunchy top virus (BBTV) as well as between the Reps of MDV and coconut foliar decay virus (CFDV). These results indicate that MDV, SCSV and FBNYV have presumably diverged from a common origin with multiple Rep genes.

References
1. Sano Y, lsogai M, Satoh S, Kojima M, 1993. Abstracts of 9th International Congress of Plant Pathology in Montreal, pp. 305.