Comparative study of the U1, U4, and U7 strains of tobacco mosaic virus

Experiments were undertaken to compare three strains of tobacco mosaic virus (TMV) belonging to the tobacco group 1 of the tobamoviruses. Although the U1, U4, and U7 strains of TMV differ significantly in virulence, it was shown that these strains are very closely related. No difference was detected in the major spots of the coat protein tryptic maps and the virions from the three strains shared the same electrophoretic mobility in agarose gels with different buffer systems. The RNA fingerprints of U1 and U7 were indistinguishable and the largest T1 omega fragments of the three strains were identical in size. However, the avirulent U4 RNA fingerprint exhibited a single spot mobility difference from the U1 and U7 RNA fingerprints. This spot corresponded to the T1 phi fragment. This unique fragment is located in the so-called l2 gene between nucleotides 976 and 992 from the 3′ end. This suggests and extends the proposal that the l2 gene is most probably implicated as a factor in the control of virulence (symptomatology).