Introduction of a NIa proteinase cleavage site between the reporter gene and HC-Pro only partially restores the biological properties of GUS- or GFP-tagged LMV.

Lettuce mosaic virus (LMV) isolates LMV-E and LMV-0 differ in their virulence on lettuce varieties carrying the mo1(2) resistance gene, which reduces viral accumulation and blocks the expression of symptoms after infection with avirulent isolates such as LMV-0. Previous work had indicated that reporter genes such as GUS or GFP affect the biological properties of recombinant LMV isolates in both susceptible and resistant lettuce varieties when fused to the N-terminus of the viral protein HC-Pro. The impact of the addition of a cleavage site for the NIa proteinase between the reporter gene and HC-Pro was evaluated, in an effort to recover the full spectrum of the biological properties of parental isolates. Symptoms, accumulation, cell-to-cell and long distance movement of the recombinant viruses containing the NIa cleavage site were studied in susceptible and mo1(2) lettuce varieties. Both LMV-0 and LMV-E recombinant viruses recovered the behaviour of their wild-type parent in susceptible plants upon addition of the NIa cleavage site. While the recombinant LMV-E modified in this way recovered the breaking properties of its wild-type counterpart in mo1(2) plants, similar modification of the LMV-0 derived recombinants failed to rescue a severe inhibition in systemic accumulation in mo1(2) plants, despite the fact that neither cell-to-cell movement nor phloem loading or unloading seemed to be severely affected.