Characterization of an intergroup serological mutant from group II RNA phage GA.

Starting from the group II RNA phage GA which has an amber mutation in the maturation protein cistron, a spontaneous mutant of group II phage GA, whose serological and electrophoretic properties became similar to those of group I phage MS2, was isolated and analyzed. The mutant has now become sensitive to anti-MS2 serum and resistant to anti-GA serum. Analysis of the nucleotide sequence of the coat protein gene revealed that G----A transition was the main change. The deduced amino acid sequence showed that five amino acids were substituted in the mutant, and three of the five became identical to MS2, resulting in increased molecular weight of the coat protein. However, it did not complement MS2. These results suggested that the serological change from group II phage GA type to group I phage MS2 type is induced spontaneously at high frequency by minor nucleotide changes in coat protein gene, and confirmed the previous results at the RNA level that MS2 and GA were related although the closeness between them seems somewhat remoter than that of groups III and IV (18, Inokuchi et al, unpublished data for the nucleotide sequence of group IV phage SP).