9604658 Gruissem Current information suggests that regulated mRNA stability is an important control point of gene expression in light-dependent development of chloroplasts and activation of photosynthesis. Recent work on mRNA processing and turnover in prokaryotes, eukaryotes and cellular organelles indicate the 3'end plays a key role in generating functional mRNA and regulating its degradation. This laboratory previously has shown that mRNA 3' stem-loop structures act in concert with nucleases and regulatory proteins to direct correct mRNA processing or degradation in chloroplasts. These nucleases are assembled into a high molecular weight (HMW) complex, which is similar in function to the recently reported RNA 'degradosome' in E. coli, but in chloroplasts is regulated by ancillary nuclear-encoded CS-type RNA binding proteins to produced correctly processed and stable mRNA 3'-ends. Plants expressing antisense RNA to the CS-type proteins die early in development consistent with the critical role of these proteins in chloroplast function. Once a stable mRNA has been formed, its degradation is determined by RNA binding proteins that regulate the endonucleolytic removal of the stabilizing 3' stem-loop structure. Degradation of the cleaved RNA is accelerated by 3' polyadenylation, which also can accelerate degradation of the mature 3' stem-loop RNA by a light-regulated mechanism. These results indicate that in plant cells polyadenylation of cytoplasmic and plastid mRNAs operates in parallel but with different consequences for the metabolism of the respective mRNAs. This research will focus on the functional dissection of the chloroplast HMW nuclease complex using component proteins which have been purified and cloned. Combined biochemical and genetic approaches will clarify the roles of enzymes that participate in mRNA metabolism and establish the function of the CS-type RNA binding proteins in light-regulated chloroplast mRNA processing and decay. Understanding the mechanisms by which pla stid mRNA decay is regulated will provide important new information on the nuclear control of organelle gene expression, and because of the importance of photosynthesis for plant productivity, will have relevance to future plant and agricultural biotechnology. Current information suggests that regulated mRNA stability is an important control point of gene expression in light-dependent development of chloroplasts and activation of photosynthesis. This research will employ combined biochemical and genetic approaches to clarify the roles of enzymes that participate in mRNA metabolism and to establish the function of RNA binding proteins in light-regulated chloroplast mRNA processing and decay. Understanding the mechanisms by which chloroplast mRNA decay is regulated will provide important new information on the nuclear control of organelle gene expression, and because of the importance of photosynthesis for plant productivity, will have relevance to future plant and agricultural biotechnology.
