Measurement of viroid sequence homology by hybridization with complementary DNA prepared in vitro.

DNA complementary to potato spindle tuber viroid (YSTV cDNA) has been used in RNA DNA hybridization experiments to study PSTV replication in a variety of hosts and to measure the amount of sequence homology between YSTV and several independently isolated viroids. When PSTV cDNA was hybridized with low molecular weight RNA isolated from PSTV-infected tomato at different times after inoculation, PSTV . PSTV cDNA hybrids were detectable before symptoms began to appear. Low molecular weight RNA was prepared from PSTV-infected tomato, tobacco, Gynura, and chrysanthemum and was analyzed by polyacrylamide gel electrophoresis and hybridization to PSTV cDNA. The identical electrophoretic mobility of PSTV isolated from each host and identical thermal stability of the corresponding RNA DNA hybrids suggest that host-specific changes in viroid sequence do not occur. Previously unreported viroids have been isolated from Columnea erythrophae and from a random selection of Solanum phureja and S. stenotomum introductions. The electrophoretic mobility of the Solanum viroids is very similar to that of PSTV, whereas the Columnea viroid and the previously reported chrysanthemum stunt viroid migrate significantly faster than PSTV. Measurement of the degree of sequence homology between PSTV and the various viroid isolates suggests that whereas the Solunum viroids are closely related to PSTV, chrysanthemum stunt and Columnea viroids are not closely related to PSTV.