A DOMINANT-NEGATIVE MUTANT OF PG13 SUPPRESSES TRANSCRIPTION FROM A CAULIFLOWER MOSAIC-VIRUS 35S TRUNCATED PROMOTER IN TRANSGENIC TOBACCO PLANTS

TGA1a and PG13 constitute a family of tobacco basic leucine zipper (bZ IP) proteins that bind to activating sequence-1 (as-1), which is one o f the multiple regulatory cis elements of the cauliflower mosaic virus (CaMV) 35S promoter. After truncation of the CaMV 35S promoter down t o position -90 (CaMV 35S [-90] promoter), transcription stringently de pends on the presence of as-1, which is recognized by nuclear DNA bind ing proteins called ASF-1. The role of the TGA1a/PG13 bZIP family in t he formation of ASF-1 and in transcriptional activation of the CaMV 35 S (-90) promoter has not yet been demonstrated in vivo We constructed transgenic tobacco plants expressing a mutant of potato PG13, which la cks its wild-type DNA binding domain. This mutant acts as a trans-domi nant inhibitor of ASF-1 formation and of expression from the CaMV 35S (-90) promoter, showing that PG13 can specifically interact with prote ins necessary for these processes. Although we did not observe any oth er obvious phenotypic changes, these transgenic plants are a potential ly valuable tool in identifying whether TGA1a and PG13 are involved in controlling promoters encoded in the plant genome.