Summary. Virus isolate Y23V, obtained from squash showing leaf curl symptoms inYunnan, China, was readily differentiated from four studied Chinese begomovirus isolates in reactions with a set of monoclonal antibodies raised against begomoviruses. The complete nucleotide sequence (2714 nts) of the DNA-Alike molecule of Y23V was determined. The DNA-A of Y23V is most closely related to that of tomato yellow leaf curl Thailand virus-[1] (TYLCTHV-[1]) (84% sequence identity). However, the AC1 and AC4 gene of Y23V DNA-A resembled to Pepper leaf curl virus from Bangladesh (PepLCBDV). The DNA-A ofY23V has three distinct regions: the region from 74-2071 nts is 95% identical to TYLCTHV-[1] excluding a 27 nt deletion; the following 386 nts are 91% identical to PepLCBDV and the rest of the DNA-A is not closely related to any reported begomovirus.Y23V, therefore, is considered to have arisen by recombination. The 84%sequence identity ofY23VwithTYLCTHV-[1] allowsY23Vto be considered as a distinct begomovirus species, for which the name squash leaf curl Yunnan virus (SLCYNV) is proposed.

Sugarcane mosaic virus (SCMV)was detected in all 62 maize samples collected from eight maize-growing provinces in China showing dwarf mosaic symptoms by immunocapture reverse-transcription polymerase chain reaction (RT-PCR). Maize dwarf mosaic virus (MDMV), Sorghum mosaic virus (SrMV) and Johnsongrass mosaic virus (JGMV), however, were not detected in any of the samples by RT-PCR. Eleven cDNA fragments of approximately 0.8 kilobases covering most of the coat protein (CP) gene of SCMV were sequenced and sequence analysis indicates that these eleven isolates share 98.1 to 100% identity at the amino acid level. Sequence comparison and phylogenetic analysis of the CP genes from the eleven Chinese isolates as well as 21SCMVsubgroup virus isolates indicate that the eleven Chinese virus isolates were closely related to SCMV with 97.0 to 98.1% sequence identity at the amino acid level, while relatively lower sequence identity was found with MDWV, SrMV or JGMV. The results indicate that the Chinese isolates are members of the SCMV species, and thus, SCMV can be considered as the most common and important potyvirus infecting maize in China.

Complete DNA-A sequences of nine Pakistani geminivirus isolates from leaf curl-affected cotton (CLCuV-PK) or from okra, and the partial sequences of several additional isolates were determined. Sequences of isolates from cotton were of four types. Isolates from leaf curl-affected okra had virtually the same sequences as those from cotton. Isolates from yellow vein mosaic-affected okra were of two types (OYVMV types 201 and 301), both distinct from but closely related to the virus isolates from cotton. Of these six types, two types of CLCuVPK are the most closely related but another (CLCuVPK type 72b) is the most distinct. Of the encoded proteins, coat protein (CP) is the most strongly conserved (92-100%amino acid sequence identity), and AC4 protein the most variable (41-87%). The 5' and 3' halves of the intergenic region of some isolates had different affinities and occurred in seven combinations, suggesting that recombination had occurred and that the origin of replication was a favoured recombination site. Similarly, the first 1520 nt of CLCuV-PK type 804a DNA resembled those of OYVMV type 301 DNA but the remaining 1224 nt were very different. The AC1 (Rep) gene and 5

Eighteen samples of begomoviruses isolated from tobacco, tomato and weed species in Yunnan, China were found to be associated with DNAb molecules, for which the complete nucleotide sequences were found to contain 1333-1355 nt. The 18 DNAb molecules identified consist of three main types, each associated with a different begomovirus species: 72-99 % nucleotide identity was found within one type, but only 39-57 % identity was found between types. All the DNAb molecules reported here and elsewhere contain a 115 nt conserved region that has 93-100 % identity with a consensus sequence, and have a common ORF encoding 118 amino acids on the complementary strand (designated C1). Coagroinoculation of the DNA-A component of Tomato yellow leaf curl China virus tobacco isolate Y10, with its associated DNAb (Y10b), shows this DNAb to be involved in symptom induction in tobacco and tomato. The in-frame ATG mutation of C1 of Y10b caused much milder symptoms as compared with wild Y10b, indicating a functional role for this ORF. Pairwise nucleotide sequence identity comparisons of DNAb molecules and their cognate viral DNA-A molecules indicate that DNAb molecules have co-evolved with their cognate helper viruses. Recombination between DNAb molecules is documented and a DNAb species concept is proposed and discussed.

The VP37 protein encoded by the RNA2 of Broad bean wilt virus 2 (BBWV2) was overexpressed in Escherichia coli. The protein was purified and a polyclonal antibody specific for the protein was produced. Time course studies byWestern blot assays in BBWV2-infected Chenopodium quinoa leaves showed that the VP37 protein was present in cells of the inoculated leaves by 12 h post inoculation and in cells of systemically-infected leaves by 2 days post inoculation. The protein was able to accumulate to a high level in infected leaves at the late infection stage. Gel retardation and UV cross-linking assays demonstrated that the VP37 protein bound preferentially single-stranded (ss) RNA and DNA in a non-sequence-specific manner. The VP37 protein-RNA complex was stable in solutions containing less than 400mM NaCl, but became fully dissociated in the solutions containing 800mM NaCl. Sequence analysis of the VP37 protein and its ability to bind ssRNA and ssDNA suggest that the protein may play a role similar to the movement proteins reported for other plant viruses.