© FORMATEX 2007 DNA replication and Pathogenecity of MYMIV Dharmendra K. Singh, Sumona Karjee, Punjab S. Malik, Nurul Islam and Sunil K. Mukherjee ∗ Plant Mol. Biol. Lab., ICGEB, AAA Marg, New Delhi 110067, India Geminiviridae is a large family of single stranded DNA plant viruses, transmitted by white fly Bamesia tabacci, and they are the major causes of huge agro-economical losses worldwide. Mungbean yellow mosaic India virus (MYMIV) belongs to the genus begomovirus, the predominant variety of geminiviruses in northern part of Indian subcontinent. It has bipartite genome, which replicates via rolling circle (RCR) model with the help of few viral and several host factors. We have focused our efforts to understand the mechanism of initiation and immediate post-initiation phases of viral RCR. The Rep-protein encoded by the virus binds to the iterons present in the origin of replication, melts the origin, and subsequently nicks a conserved specific sequence of DNA, the region from where the RCR begins. The Rep protein also carries out the displacement of the parental strand required for the passage of replication fork. We have identified few host factors that act as accessory proteins of Rep during these phases of replication as mentioned. The high phyto-pathogenicity of the virus may be attributed to its potential to suppress host antiviral response, i.e., RNA silencing. We have developed various assays to identify viral RNAi suppressors. The AC2 suppressor protein of the MYMIV has been characterized for its various activities and its suppression domain has been mapped. This review will primarily focus upon the understanding the mechanistic aspect of initiation of MYMIV replication, the biochemical activity of the replication in relation to host as well as viral factors; and the RNA silencing suppressor activity of the virus protein AC2 and its potential applications. Key words: Geminivirus; replication; MYMIV; AC2; RNA silencing. Introduction Geminiviruses constitute a large family of phytopathogens (Geminiviridae) having single stranded circular genome and vector- (white fly, Bemisia tabaci) mediated transmissibility. These viruses devastate some of the economically important plants ranging from dicots to monocots and impose huge agro-economic losses worldwide. The family geminiviridae have been classified into four genera, namely Begomovirus, Curtovirus, Topocuvirus and Mastrevirus, depending on their genomes, mode of transmission and host range [1]. The genus begomoviridae generally comprises of bipartite genome (two components, namely DNA- ‘A’ and ‘B’), which are transmitted by white fly. Mungbean yellow mosaic India virus (MYMIV) is a representative of the genus begomovirus, which is prevalent in northern part of Indian subcontinent causing yellow mosaic disease (YMD) [2]. The most affected leguminous crops are Cajanus cajan, Glycine max, Phaseolus aconitifolius, P. aureus, P. vulgaris ‘French bean’, Vigna mungo. The component ‘A’ of MYMIV encodes for proteins important for viral replication and en-capsidation whereas DNA ‘B’ component mainly codes for proteins important for intracellular movement (BL1, BV1) and transport of viral ssDNA. The two components share a region of high sequence homology that is known as ‘CR’, the place from where the replication of the viral DNA genomes initiates. In order to ∗ Corresponding author: e-mail: sunilm@icgeb.res.in, Phone: +91-11-26181242 Communicating Current Research and Educational Topics and Trends in Applied Microbiology A. Méndez-Vilas (Ed.) 155 ©FORMATEX 2007 _____________________________________________________________________