In order to clarify the taxonomic status of an RNA coliphage, MX1 (a serological intermediate between groups III and IV), we examined (i) read-through protein synthesis in a cell-free protein-synthesizing system, (ii) the peptide map of the coat protein and (iii) the RNA sequence in the 3′-terminal region of MX1 RNA. We found that the characteristics of MX1 were closer to those of group III phages than to those of group IV. For example, the gel electrophoretic pattern of the protein coded for by MX1 RNA was the same as that of group III phage proteins. Peptide fingerprints of the coat protein of MX1 showed that seven tryptic peptides overlapped with corresponding peptides of Qβ (group III phage), whereas only two peptides overlapped with those of SP (group IV phage). Furthermore, in the base sequence of the first 200 nucleotides from the 3′-end of MX1 RNA, about 70% of the nucleotides were homologous to those of the Qβ RNA, whereas the homology to SP RNA was only 53%. These results suggest that MX1 is more closely related to group III phages than to group IV phages and we propose that it should be assigned to the former group.


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