ROLE OF THE VARIABLE DOMAIN IN MODULATING POTATO SPINDLE TUBER VIROIDREPLICATION

Potato spindle tuber viroid (PSTVd) is believed to undergo a series of specific structural transitions during replication. The variable doma in of PSTVd is known to contain sequences that are important for repli cation/accumulation as well as one of three ''premelting regions'' whi ch control breakdown of the native structure in vitro. We have examine d the structural and biological effects of five single and two double nucleotide substitutions within premelting region 3 in an effort to is olate temperature-sensitive mutations affecting PSTVd replication or p athogenesis. None of these mutants replicated as rapidly as the wild t ype, and a variety of spontaneous sequence changes were detected in th eir progeny, Higher temperatures were able to partially overcome the i nhibition of replication associated with a more stable secondary struc ture, but no well-defined temperature-sensitive PSTVd mutants were ide ntified. Selective pressures arising from the interaction of assay tem perature and structural stability in vivo appear capable of moving PST Vd populations between peaks of relatively high fitness. Depending on the exact nature and location of the mutation, selection may occur at the level of either the plus or the minus strand. (C) 1996 Academic Pr ess, Inc.