Avocado sunblotch viroid: primary sequence and proposed secondary structure.

The sequence of the 247 nucleotide residues of the single strand circular RNA of avocado sunblotch viroid (ASBV) was determined using partial enzymic cleavage methods on overlapping viroid fragments obtained by partial ribonuclease digestion followed by 32p-labelling in vitro at their 5'-ends. ASBV is much smaller than potato spindle tuber viroid (PSTV; 359 residues) and chrysanthemum stunt viroid (CSV; 356 residues). A secondary structure model for ASBV is proposed and contains 67% of its residues base paired. In contrast to the extensive (69%) sequence homology of CSV with PSTV, only 18% of the ASBV sequence is homologous to PSTV and CSV. There are eight potential polypeptide translation products with chain lengths from 4 to 63 amino acid residues coded for by the plus (infectious) strand and four potential translation products (2 to 60 residues) coded for by the minus strand. An improved method is described for the synthesis of gamma-32p-ATP of high specific activity.