Agrobacterium-mediated gene transfer was used to co-express three virus-derived post-transcriptional gene silencing (PTGS) suppressors, P19 from tomato bushy stunt virus and two species of helper component proteinase (HcPro) from tobacco etch virus (TEV) and turnip mosaic virus, with beta-glucuronidase (GUS) in harvested lettuce leaf tissue to investigate whether GUS accumulation increases in the presence of PTGS suppressors. Co-expression incubations were 3-5 days at 4 and 22 degrees C. GUS activity and leaf viability were measured after incubation. Co-expression of PTGS suppressors did not elevate GUS expression levels. Under certain incubation conditions, co-expression of TEV HcPro significantly lowered transient GUS expression and was detrimental to leaf viability, suggesting that expression of PTGS silencers may have a negative effect on transient expression levels that outweighs any effects of PTGS suppression in harvested leaf tissues.