Viability and pathogenicity of intersubgroup viroid chimeras suggest possible involvement of the terminal right region in replication.

To investigate the structural determinants regulating viroid replication and pathogenicity, we have examined the biological properties of four chimeric viroids containing sequences derived from hop stunt (HSVd) and citrus exocortis (CEVd) viroids. The viability of each chimera--CEHS (CEVd left half + HSVd right half), HSCE (HSVd left half + CEVd right half), CE/HS-TR (CEVd + HSVd right terminal loop), and HS/CE-TR (HSVd + CEVd right terminal loop)--was tested by inoculation onto cucumber and tomato seedlings. Chimeras CEHS and HSCE were not infectious, but CE/HS-TR and HS/CE-TR replicated stably and produced disease symptoms when inoculated onto tomato or cucumber, respectively. Progeny accumulation was reduced 10-fold or more compared to that of CEVd in tomato or HSVd in cucumber. The results suggested that the TR, like the TL and P regions, forms a relatively independent structural unit that contributes to the total function of a viroid. The effect of sequences in the right terminal loop on pathogenicity appears to be indirect, modulating the efficiency of viroid replication (or accumulation) efficiency rather than symptom expression per se.