In transgenic tobacco, pea Ferredoxin-1 (Fed-1) mRNA accumulates rapidly in response to photosynthesis even when the transgene is driven by a constitutive promoter. To investigate the role of photosynthesis on Fed-1 mRNA stability, we used the tetracycline repressible Top10 promoter system to specifically shut off transcription of the Fed-1 transgene. The Fed-1 mRNA has a half-life of approximately 2.4 hr in the light and a half-life of only 1.2 hr in the dark or in the presence of the photosynthetic electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). These data indicate that cessation of photosynthesis, either by darkness or DCMU results in a destabilization of the Fed-1 mRNA. Furthermore, the Fed-1 mRNA half-life is reduced immediately upon transfer to darkness, suggesting that Fed-1 mRNA destabilization is a primary response to photosynthesis rather than a secondary response to long-term dark adaptation. Finally, the two different methods for efficient tetracycline delivery reported here generally should be useful for half-life measurements of other mRNAs in whole plants.

In previous studies using leaves of light-grown transgenic tobacco plants, we have shown that sequences located within the transcribed region of the pea Fed-1 gene (encoding ferredoxin I) are major cis-acting determinants of light-regulated mRNA accumulation. However, we show here that these internal sequences are less important for the Fed-1 light response in etiolated tobacco seedlings than they are in green leaves and that upstream elements confer organ specificity and contribute significantly to Fed-1 light responses in etiolated material. Light effects mediated by upstream response elements are thus most pronounced during the initial induction of gene activity, whereas internal elements play a more prominent role in modulating Fed-1 expression once the gene is already active.

In previous studies using leaves of light-grown transgenic tobacco plants, we have shown that sequences located within the transcribed region of the pea Fed-l gene (encoding ferredoxin I) are major cis-acting determinants of light-regulated mRNA accumulation. However, we show here that these internal sequences are less important for the Fed-l light response in etiolated tobacco seedlings than they are in green leaves and that upstream elements confer organ specificity and contribute significantly to Fed-7 light responses in etiolated material. Light effects mediated by upstream response elements are thus most pronounced during the initial induction of gene activity, whereas internal elements play a more prominent role in modulating Fed-l expression once the gene is already active.

We monitored Fed-7 (encoding ferredoxin I) mRNA levels in etiolated transgenic tobacco seedlings containing the intact pea Fed-7 gene to determine if the characteristic light responses of this gene in pea seedlings are also observed in transgenic tobacco. Fed-7 transcript levels in transgenic tobacco seedlings closely paralleled those of the native gene in pea buds when etiolated seedlings were transferred to white light. However, the response to red light was much smaller in tobacco than in pea and was not efficiently reversed by far-red light. The red light response of endogenous tobacco ferredoxin transcripts is closely comparable to that of the Fed-7 transgene, with a similar lack of photoreversibility. Thus, the pea Fed-7 transgene responds normally to tobacco gene-regulatory factors, but these factors are less influenced by phytochrome in tobacco cotyledons than in pea buds.