Gamma ray-induced small plaque mutants of western equine encephalitis virus.

Small plaque mutants of Western equine encephalitis virus were obtained from the surviving fractions of wild-type virus which was irradiated with gamma rays. The frequency with which small plaque mutants appeared in the surviving fraction increased with the radiation dose. These mutants were not more resistant to radiation than wild-type virus. The growth rate of a mutant, S127, was lower than that of wild-type. Clonally purified mutant virions presented two peaks in a velocity sedimentation profile; peak 1 corresponded to the peak of wild type and peak 2 moved faster than peak 1. Virions of both peaks were infectious and consistently formed small plaques in chicken embryo cells. Virions reisolated from either peak and grown in chicken embryo cells also revealed two peaks in sedimentation analysis. In the electron microscope examination peak 2 proved to consist of giant form particles, each of which contained more than one nucleoid surrounded with a common envelope. Despite this remarkable morphological difference, densities of the wild-type and S127 mutant virions were similar in cesium chloride gradients. The RNAs and proteins of mutant virions could not be distinguished from those of wild type on the basis of size or charge.