Isolation and Basic Characterization of Temperature‐sensitive Mutants From Semliki Forest Virus

Sixteen temperature‐sensitive mutants were isolated from N‐methyl‐N'‐nitro‐N‐nitrosoguanidine treated Semliki Forest virus. Seven of the mutants were unable to synthesize virus specific RNA at the nonpermissive temperature (39° C) and were classified as RNA mutants; as evidenced by temperature shift‐up experiments they had defects only in early functions. Two mutants, designated RNA ±, made less virus specific RNA than the wild type virus. Six RNA+ mutants synthesized 42 S and 26 S RNA at 39° C in almost the same amounts and ratio as the wild type. Only one of them, ts‐3, was unable to form the viral 140 S nucleocapsid at the nonpermissive temperature. One of the RNA+ mutants, ts‐1, made a higher proportion of 42 S RNA, presumably at the expense of 26 S RNA, both at 27° C and at 39° C. The synthesis of 42 S RNA and 26 S RNA in the Semliki Forest and Sindbis wild type virus infected cells was temperature dependent. There was an increased synthesis of 42 S RNA compared to 26 S RNA at 39° C, the situation being reversed at 27° C.