Molecular characterization of the Rep protein of the blackgram isolate of Indian mungbean yellow mosaic virus

The complete nucleotide sequence of the blackgram isolate of mungbean yello w mosaic virus, IMYMV-Bg, which infects legumes in India, was determined an d compared at the amino acid level with those of other whitefly-transmitted geminiviruses. The genome organization of IMYMV-Bg was similar to that of the begomoviruses. A unique feature of the genome organization was the sequ ence divergence of the common region (CR) between DNA-A and DNA-B. In order to understand the mechanism of viral DNA replication, the replication init iator protein, Rep, of IMYMV-Bg was overexpressed in E. coli. The recombina nt and refolded Rep bound to CR-sequences of IMYMV-Bg in a specific manner. In this study, evidence is presented for ATP-upregulated cleavage function and ATP-mediated conformational change of Rep. It is hypothesized that, al though ATP is not required for cleavage, ATP-mediated conformational change s may result in better access of Rep to the DNA-cleavage site. Evidence is also presented for a site-specific topoisomerase function of Rep, which has not been demonstrated before. The Rep protein can be classified as a type- 1 topoisomerase because of its nicking activity and sensitivity towards cam ptothecin, a topoisomerase type-1 inhibitor.