A mutant of Sindbis virus which is released efficiently from cells maintained in low ionic strength medium.

Experiments many years ago showed that reducing the ionic strength of the medium Sindbis virus-infected chick embryo fibroblasts were maintained prevented the release of progeny virus into the medium. It was proposed at that time that the susceptibility of virus release to inhibition by low ionic strength (LIS) medium might be determined by the properties of the viral envelope proteins. We thought that if such were the case, it might be possible to obtain a viral mutant which is resistant to the inhibitory effect of LIS medium. We report here that we were able to isolate such a mutant of Sindbis virus, and that we identified two amino acids (Glu23 and Thr76) which, when present in the E2 protein, made possible the release of infectious progeny virus from cells incubated in LIS medium.