| Geminivirus is a group of plant DNA viruses with single strand DNAs as genomes and twinned icosahedra as particle morphology. It occurrs world wide and causes serious disease on various species of both monocotyledonous and dicotyledonous plants. In China, many disease that caused by geminiviruses had been reported in Yunnan, Guangxi, Hainan, Guangdong and Taiwan provinces. However, no report of Geminivirus has been made in Fujian, which also belong to south-China.In this study, a disease infected by Begomovirus on tobacco plants (Nicotiana tabacum), which displaying vein swelling, mild enation and downward leaf curling symptoms, was found in the Jinshan district of Fuzhou, Fujian. PCR using general primers for specific fragments of Begomovirus were carried out on 4 symptomatic samples (F1, F2, F3 and F4) . The sequencing results of PCR products (EF531601～3 and partial sequence of EF527823) showed they were duplicate clones, then F2 sample was selected for further research. The complete sequence of DNA-A of F2 isolate was soon determined, comprising of 2737 nucleotides (Acc. No. EF527823) . Attempts made to isolate DNA-B component were unsuccessful. However, a novel molecule (designated as DNA-β) was found to be associated with F2, which consists of 1336 nucleotides (Acc. No. EF527824) . Sequence comparison analysis with the complete DNA-A sequence of F2 reveals that it had a homology as high as 95．7 % to Ageratum yellow vein virus (AYVV, Acc. No. X74516) from Singapore, indicating that F2 is an isolate of AYVV infecting tobacco in Fujian province, and the name Ageratum yellow vein virus-F2 isolate is proposed. The sequence of DNA-βcomponent of F2 was also obtained, it consists of 1345 nucleotides, sharing the highest nucleotide sequence identity (97．2 %) with the DNA (3 of Tomato leaf curl virus (ToLCV, Acc. No. AJ542495) from Taiwan and 88．8 % sequence identity with AYVVβ(Acc. No. AJ252072) . Further analysis on the encoding open reading frames (ORFs) on both DNA-A and DNA-βcomponents showed that F2 genomes encoded 8 ORFs, withl ORF (βC1) encoded on the complemental strand of DNA-β, and 2 ORFs (V1 and V2) on the viral strand while 5 ORFs (C1, C2, C3, C4 and C5) on the complemental strand of DNA-A, within which a novel ORF C5 was found. C5 gene has a 297 base pairs (bp) ORF that coding a 98 amino acid protein, and its function was still unknown.In an attempt to confirm the pathogenecity and biological role of F2 C5 protein, a partial promoter C5P, with a size of 512 bp fragment in the upstream of the translating initiation site ATG of F2 C5 gene was cloned. Then the previously contained CaMV 35S promoter of plant expression vector pCAMBIA-1301 was removed and replaced by inserted the partial promoter C5P in the same site. As a result, a recombinated vector pCAMBIA-C5P with C5P as promoter and the gus gene as a reporter gene was constructed. Promoter activity of C5 ORF of F2 was identified by Agrobacterium-mediated transient expression. Histochemical assay revealed that just as positive control pCAMBIA-1301, pCAMBIA-C5P in which C5P serves as the promoter of gus gene also has the ability to cleave the substrate X-Gluc to produce the insoluble blue precipitate dichloro-dibromoindigo. According to this result, a conclution was made that C5P does have promoter activity, this make it possible for the transcription and expression of C5 gene in diseased cells. In order to detect the existence of F2 C5 protein in symptomatic plant cells, prokaryotic expression of F2 C5 protein was done in Escherichia coli BL21(DE3) and polyclonal rabbit antibody was then obtained by immunization.In addition, Vector transmission studies were conducted using whitefly (Bemisia tabaci) as vector of Begomovirus on non-infected Nicotiana tabacum, Nicotiana glutinosa, Ageratum conyzoides, Oxalis corymbosa, and Phyllanthus urinaria to investigate the host range of the pathogen. The results of this assay showed that some species of Solanaceae, Compositae, Euphorbiaceae, Oxalidaceae plants are all susceptive host of F2 isolate. Further assay indicated that F2 may evolved by recombination of AYVV and ToLCV during the co-infection progress of these two viruses.In recent years, many regulating factors have been identified. Based on the prediction on the function of AC5/C5 protein, it is likely that AC5/C5 protein may be a key regulator and plays a significant role in certain stages of virus life cycle. In view of the possible functions of AC5/C5 protein contributing to the pathogenecity of viruses, it is of far reaching importance to investigate the interaction relationship of AC5/C5 protein with other virus-coding proteins or host factors, which will undoubtedly avail to clarify the mechanism of Geminivirus participating in plant cell cycle and growth regulation, and subsequently improving crop management and developing new strategy on plant diseases control.
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