The baculovirus multicapsid nucleopolyhedrovirus (MNPV) has high potential for development as a bio-insecticide for control of the beet armyworm (). It is highly infectious for larvae and its host range is very narrow. A prerequisite for such application is the possibility of growing this virus in large quantities, e.g. in insect cell lines. It was observed, however, that polyhedra of SeMNPV plaque-purified in SeUCR1 cells did not cause larval mortality or morbidity when fed to larvae. As this suggested a genetic alteration in produced SeMNPV, comparative restriction analysis of and produced SeMNPV DNA was performed. The restriction patterns of viral DNa from several different plaques always differed from that of the wild-type in the same way, suggesting that a large, single deletion had occurred in the produced viral genome. In order to localize this deletion more precisely a detailed physical map of the wild-type SeMNPV genome was constructed, using the restriction endonucleases I, HI, II, I, I, dIII and I. In addition, the entire SeMNPV genome was cloned into a library containing five overlapping cosmids and a plasmid library. About 80 restriction sites were located and the orientation of the map was set according to the location of the polyhedrin and p 10 genes. The approximate size of the viral genome was 134 kbp. Based on this map it could be established that mutant SeMNPV, obtained by passage in cell culture, contained a single deletion of approximately 25 kbp between map units 12·9 and 32·3.


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