Constructs previously described that were used in this study include: T100, the wt TBSV genome construct (Hearne et al, 1990 ); AS1m1 and Psg1-S5, modified TBSV genomes with 3-nt substitutions in AS1 (Choi and White, 2002) or five silent mutations in RS1 (Zhang et al, 1999) , respectively; a TBSV replicon corresponding to a prototypical TBSV DI RNA (Wu et al, 2001 ) and HL47, a replicon containing a sg RNA promoter as its plus-strand promoter (Lin and White, 2004) .