Mutants of sindbis virus. IV. Heterotypic complementation and phenotypic mixing between temperature-sensitive mutants and wild-type Sindbis and Western equine encephalitis viruses.

Heterotypic complementation between temperature-sensitive (ts) mutants of Sindbis (SIN) and Western equine encephalitis (WEE) viruses occurs under appropriate conditions. One heterotypic pair, SIN ts153 X WEE ts39 showed efficient complementation, and four other combinations gave detectable complementation, indicating that these two viruses, which are closely related serologically and biochemically, are sufficiently closely related to complement each other functionally. Cells mixedly infected with ts mutants or wild-type strains of both SIN and WEE viruses produced phenotypically mixed virions, in addition to both parental viruses. Various types of phenotypically mixed virions have been identified by neutralization with corresponding antisera, by thermal inactivation and by temperature sensitivity of replication. Some virions contained WEE genomes and envelopes containing primarily SIN proteins. Other phenotypically mixed virus yields contained primarily doubly neutralizable viruses which are presumed to have a mosaic of envelope proteins. Phenotypically mixed virions were morphologically indistinguishable from the parental types.