Unusually Large-Plaque Mutant of Sindbis Virus

Summary When Sindbis virus plaques were formed in chick embryo fibroblasts (CEF) at 40°C, plaques varying in size were observed. Plaques measuring less than 2 mm (sp) were subjected to cloning. Out of 51 sp plaques so examined, three yielded predominantly, on passage through CEF, unusually large-plaque (u) mutants. This mutant as well as the parent large-plaque (lp) and sp viruses were purified by consecutive clonings. Mixed infection of CEF with the sp and u viruses tended to exclude the sp virus. Contrarily, mixed infection with the lp virus and the u mutant tended to exclude the latter. Thus for maintenance of the u mutant, frequent clonings had to be done, because otherwise an incidental appearance of the parent type virus by back mutation seemed to result in exlusion of the u mutant virus. The u mutant as well as the sp virus was more sensitive to interferon than the lp virus. The reason why the interferon-sensitive u mutant can grow more rapidly and yield a higher amount of virus than the parent lp virus remains to be elucidated.