T7 RNA polymerase drives transcription of a reporter gene from T7 promoter, but engenders post-transcriptional silencing of expression

Transgenic plants expressing T7 RNA polymerase were crossed with other tran sgenic plants carrying the reporter gene (GUS) placed under the control of the T7 promoter. Twenty one lines of the double-transformed plants (altoget her 2100 plants) expressed T7 RNA polymerase but none expressed GUS. Howeve r, GUS expression was found in pollen grains and callus in only two cases, indicating that T7-driven correct expression in tobacco can occur. Indeed, nuclear run-on experiments demonstrated the presence of a considerable amou nt of GUS transcripts in the nucleus, indicating that the T7 RNA polymerase is active. However, GUS activity could not be detected in leaves of the do uble-transformed plants by histochemical or enzymatic assays, and RNase pro tection assays indicated the absence of GUS mRNA. GUS expression is, theref ore, down-regulated at the post-transcriptional level. A silenced double-tr ansformed plant was crossed with an expressing 35S-GUS plant. All the tripl e-transformed plants did not express GUS, indicating that the T7-derived ge ne silencing acts in trans and can cause the silencing of an active gene. ( C) 1999 Elsevier Science Ireland Ltd. All rights reserved.