Specific sequence changes in the 5'-terminal region of the genome of satellite tobacco mosaic virus are required for adaptation to tobacco mosaic virus.

The genome of satellite tobacco mosaic virus (STMV) adapted to tobacco mosaic virus (TMV), tomato mosaic virus or green tomato atypical mosaic virus consistently had two single base deletions at positions 1 and 61, corresponding to bases A and G, respectively, as compared to the type-strain genome which is naturally adapted to tobacco mild green mosaic virus (TMGMV). Transcript RNAs (STMV(TMV)) from clone pSTMV(TMV) which captured the deletions at positions 1 and 61 were infectious when co-inoculated to tobacco plants with either TMV or TMGMV at infection frequencies of > 90%. Two new STMV variants were created to investigate whether both deletions were essential for adaptation to TMV. These were STMV(TMGMV) deltaA1, which had the A at position 1 (A1) deleted, and STMV(TMGMV) deltaG61, which lacked G61. STMV(TMGMV) deltaA1 was infectious (75% frequency) in the presence of either TMV or TMGMV. Virion RNA of STMV(TMGMV) deltaA1 lost G61 after one infection cycle with TMV. This deletion did not occur in co-infections with TMGMV. STMV(TMGMV) deltaG61, like the clone STMV(TMGMV), was infectious (100% frequency) with TMGMV but TMV did not support this clone. When Nicotiana benthamiana protoplasts were transfected with STMV(TMGMV), STMV(TMGMV) deltaA1 or STMV(TMV), STMV replicated when TMGMV was the helper virus. STMV(TMV) and STMV(TMGMV) deltaA1 replicated in the presence of helper TMV, but STMV(TMGMV) did not, the same result as in whole plants. The deletion of A1 is thus essential for initial STMV adaptation to TMV and the eventual deletion of G61 is a predicted additional change.