Sense- and antisense-mediated gene silencing in tobacco is inhibited by the same viral suppressors and is associated with accumulation of small RNAs

Antisense-mediated gene silencing (ASGS) and posttranscriptional gene silen cing (PTGS) with sense transgenes markedly reduce the steady-state mRNA lev els of endogenous genes similar in transcribed sequence. RNase protection a ssays established that silencing in tobacco plants transformed with plant-d efense-related class I sense and antisense chitinase (CHN) transgenes is at the posttranscriptional level. Infection of tobacco plants with cucumber m osaic virus strain FN and a necrotizing strain of potato virus Y, but not w ith potato virus X, effectively suppressed PTGS and ASGS of both the transg enes and homologous endogenes. This suggests that ASGS and PTGS share compo nents associated with initiation and maintenance of the silent state. Small , ca, 25-nt RNAs (smRNA) of both polarities were associated with PTGS and A SGS in CHN transformants as reported for PTGS in other transgenic plants an d for RNA interference in Drosophila, Similar results were obtained with an antisense class I beta -1,3-glucanase transformant showing that viral supp ression and smRNAs are a more general feature of ASGS, Several current mode ls hold that diverse signals lead to production of double-stranded RNAs, wh ich are processed to smRNAs that then trigger PTGS. Our results provide dir ect evidence for mechanistic links between ASGS and PTGS and suggest that A SGS could join a common PTGS pathway at the double-stranded RNA step.