Poster: Transcription Regulation

Abs # 1030: Silencing of homologous transgenes: TGS or PTGS?

Presenter:	Yang, Guojun , guojun@idmb.tamu.edu
Authors	Yang, Guojun  (A)   Lee, Yeon-Hee  (B)   Jiang, Yiming  (A)   Dong, Jinjiang  (A)   Kumpatla, Siva P (C)   Hall, Timothy C (A)
Affiliations:	(A): IDMB & Biology Department, Texas A&M University (B): Plant Biotechnology, NIAB, Suwon, korea. (C): Department of Trait Genetics & Technologies,Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, IN 46268

The presence of identical DNA sequences in plants and other organisms has been the focus for studies on homology dependent transcriptional gene silencing (HDTGS). Interaction (pairing) of multiple copies of transgenes has often been considered to be the major factor for transgene silencing. To further study the possible role of pairing between identical genes in silencing, we supertransformed a well characterized homozygous silenced rice line harboring mUbi-bar and 35S-BttcryIIIA transgenes with a binary vector bearing mUbi-bar and 35S-uidA genes. Surprisingly, the incoming mUbi transgenic promoter was not silenced whereas the incoming 35S transgenic promoter was silenced. These results indicate that transcriptional silencing is not necessarily induced by pairing of homologous sequences. Further evidence in support of this concept was obtained by analysis of hybrids derived from crosses between a 35S-uidA silenced line and a 35S-uidA expressing line. If HDTGS is a prevalent event, it follows that GUS expression should be silenced in hybrid progeny bearing both transgenes, this did not occur. Thus, results of experiments using both sexual and somatic approaches provide persuasive evidence that HDTGS does not arise from homology per se. Rather, sequence rearrangements may yield aberrant transcripts, thereby establishing RNAi-based post-transcriptional silencing (PTGS). Experiments to detect small (21-26 nt) RNAs corresponding to the silenced promoters are underway, with the prediction that small RNAs corresponding to 35S promoter will be detected in the supertransformed lines but not in the crossed lines. Supported by NSF MCB-0110477 and Dow Agrosciences LLC.