The proposed experimentation will isolate and characterize the arabidopsis gene encoding NADPH- protochlorophyllide oxioreductase (PCR). This appears to be a nuclear gene whose product is found in immature chloroplasts (etioplasts). The level of gene product in the cell is light-regulated, as is that of a number of other gene products found in the chloroplast. A novel feature of this gene is that the amount of product is decreased by exposure to light, whereas expression of most other light-regulated genes so far studied is augmented by light. Timko will isolate the structural gene for PCR, then determine its sequence and the start sites for its transcription. He will use plant DNA transformation techniques to define the regulatory elements of the gene, including the photoresponsive element. The element will be used to isolate and characterize the cellular factor(s) to which it responds. The regulation of plant gene expression by light has been a central problem in plant molecular biology. The proposed work will examine the structure and regulation of a novel photoregulated gene; one whose expression appears to be inhibited by light. The gene's product acts at an important step in the light-induced maturation of the chloroplast. Thus, the proposed work should lead to a valuable increase in our understanding of the regulation of plant gene expression and of the morphogenesis of the chloroplast.
