Abstract
Summary
The unusually large-plaque (u) mutant previously isolated from a small-plaque clone of Sindbis virus was maintained first by cloning passage, because the ordinary passage resulted in replacement of the u mutant by the more interferon-resistant parent type virus. After 9 cloning passages, it was found that the virus did not form such large plaques at 40 ° as it did before, and the pfu titer determined at 40 ° was lower than that determined at 37 °. However, serial passage of this virus in a standard passage manner adopting incubation at 40 ° allowed it to regain the original plaque morphology, i.e., formation of very large plaques at 40 ° as well as at 37 ° with about equal plating efficiencies. It seems likely that back mutation toward the parent type seldom occurs at 40 °. The virus so maintained was found to be slightly more sensitive to interferon, less pathogenic for intracerebrally inoculated baby mice, and more resistant to agar inhibitor than the parent type virus.
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