Oligomeric potato spindle tuber viroid (PSTV) RNA does not process autocatalytically under conditions where other RNAs do.

In vitro-synthesized oligomeric linear RNAs representing the replicative intermediates of potato spindle tuber viroid (PSTV) were subjected to a large variety of in vitro conditions where self-splicing of group I introns occurs, and where self-cleavage and self-circularization of the satellite RNA from tobacco ringspot virus particles and self-cleavage of oligomeric forms of avocado sunblotch viroid RNA has been observed. No evidence whatsoever could be obtained that the PSTV RNA oligomers monomerize and subsequently circularize autocatalytically in vitro under these conditions. These results are at variance with the low level of self-cleavage of dimeric PSTV RNA reported by H. D. Robertson et al. Virology 142, 441-447, 1985). A comparable and seemingly autocatalytic in vitro cleavage we have initially observed under certain conditions could be related to an unknown heat-stable RNase contaminating a batch of autoclaved ultrapure ammonium sulfate used in the corresponding incubating mixtures as additive.