Minor differences between nucleotide sequences of mild and severe strains of potato spindle tuber viroid

VIROIDS are the smallest replicating pathogenic agents known 1 . Composed entirely of RNA, they have genome sizes in the range of 330–380 nucleotides 2 , 10 times smaller than the smallest bacteriophage of Escherichia coli 3 . Although there is no evidence for the existence of a protective protein coat for viroids, they infect a wide variety of plants and in many produce severe disease symptoms 1,4 . The molecular mechanisms by which viroids replicate and interact with their hosts are not yet understood. In its most severe form, the disease 5,6 caused by potato spindle tuber viroid (PSTV) causes general stunting of potato plant growth, deformity of the upper foliage, and production of disfigured potatoes 5 . Mild strains of PSTV which produce barely detectable symptoms have also been isolated 7 . Furthermore, plants infected by mild strains are somehow protected from developing symptoms following subsequent inoculation with severe strains 8,9 . This biological protection phenomenon has been called cross-protection 7 . As the only known component of viroids is RNA, mild and severe strains must differ in nucleotide sequence. The study described here aimed to determine the extent of this difference.