9507396 Thompson This project concerns the expression of Fed-l, a nuclear gene encoding chloroplast ferredoxin in higher plant leaves. Fed-l mRNA accumulates in response to illumination, both in dark-grown seedlings and in photosynthesizing leaves of green plants. However, the response mechanism in green leaves differs dramatically from the promoter-mediated transcriptional responses typical of most other plant genes. Previous work has focused on the response in green leaves and identified an internal light response element (iLRE) within the transcription unit. Sequences located both in the untranslated leader and in the 5' portion of the coding region are essential for iLRE function, and an open reading frame is required in this portion of the coding region. These observations support the hypothesis that light effects are mediated by translation-dependent changes in mRNA stability. Such mechanisms are only beginning to be studied in plants, but are likely to be of considerable importance. Experiments are designed to provide additional information about the sequence requirements for iLRE function, and to test the hypothesis that the Fed-l light response involves changes in mRNA transport and/or degradation. Components of the iLRE will be defined at the nucleotide level and additional nonsense codon insertions will be tested in order to determine whether an open reading frame must extend over the full length of the coding region. Chemically inducible and repressible promoters will be used to determine the in vivo half life of Fed-l mRNA in the absence of inhibitors and to test for light effects on the kinetics of mRNA accumulation in nuclear and cytoplasmic compartments. Our experiments will provide a basis for future investigations, including a genetic analysis of mutations affecting Fed-l expression, and will help us reach our long term goal of defining the entire set of events and factors that regulate production of this important photosynthetic protein. %%% This project concerns the e xpression of Fed-l, a nuclear gene encoding chloroplast ferredoxin in higher plant leaves. Fed-l mRNA accumulates in response to illumination, both in dark-grown seedlings and in photosynthesizing leaves of green plants. This research will investigate the hypothesis that light effects are mediated by translation-dependent changes in mRNA stability. Such mechanisms are only beginning to be studied in plants, but are likely to be of considerable importance. Our experiments will provide a basis for future investigations, including a genetic analysis of mutations affecting Fed-l expression, and will help us reach our long term goal of defining the entire set of events and factors that regulate production of this important photosynthetic protein. ***
