Forty-six temperature-sensitive mutants of bacteriophage φ6 were isolated. Crosses between these mutants yielded a pattern consistent with three segregation groups for the mutant genes. Nonsense mutants were also isolated and these fell into eight classes on the basis of proteins missing during infection of nonsuppressor cells. Crosses between these mutants also showed three classes of segregation groups and these classes were shown to be the same as those found for the temperature-sensitive mutants. The three sets of segregation groups were as follows: Set A included mutants affecting polymerase factors PI and P2. Set B included mutants affecting adsorption proteins P3 and P6 as well as the host range marker hl. Set C included mutants affecting the synthesis of proteins P8, P12, P9, P5, and Pll. The three segregation groups probably correspond to the three segments of dsRNA found in virions and infected cells. On the basis of the size of proteins Pl and P2, we conclude that set A must be localized on the largest segment of dsRNA.
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