THE FREQUENCY AND DEGREE OF COSUPPRESSION BY SENSE CHALCONE SYNTHASE TRANSGENES ARE DEPENDENT ON TRANSGENE PROMOTER STRENGTH AND ARE REDUCED BY PREMATURE NONSENSE CODONS IN THE TRANSGENE CODING SEQUENCE

By comparing the effects of strong and weak promoters that drive sense chalcone synthase (Chs) transgenes in large populations of independen tly transformed plants, we show here that a strong transgene promoter is required for high-frequency cosuppression of Chs genes and for prod uction of the full range of cosuppression phenotypes. In addition, sen se Chs transgenes driven by a cauliflower mosaic virus 35S promoter po ssessing a single copy of the upstream activator region (UAR) were fou nd to produce a significantly lower degree of cosuppression than they did when the transgene promoter possessed two or four copies of the UA R, It has been shown elsewhere that 35S promoter strength increases wi th increasing UAR copy number. Frameshift mutations producing early no nsense codons in the Chs transgene were found to reduce the frequency and the degree of cosuppression, These results suggest that promoter s trength and transcript stability determine the degree of cosuppression , supporting the hypothesis that sense cosuppression is a response to the accumulation ct transcripts at high concentrations. This conclusio n was shown to apply to single-copy transgenes but not necessarily to inversely repeated transgenes. The results presented here also have si gnificance for efficient engineering of cosuppression phenotypes for u se in research and agriculture.