This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Many of the factors necessary for proper plastid development are actually encoded in the plant's nuclear genome. Furthermore a growing the body of evidence suggests that the coordinated expression of these genes depends in part on the functional state of the chloroplast. For example, nuclear mutations that result in developmentally arrested chloroplasts also result in the reduced expression of nuclear-localized photosynthetic genes. This plastid-derived signaling cascade is essential for coordination of both nuclear and chloroplast localized genes that encode components of photosynthesis. Arabidopsis GUN (genomes uncoupled) loci have been identified as components of the plastid to nucleus signaling pathway. There are currently, five known genes within the GUN loci (GUN1, GUN2, GUN3, GUN4 and GUN5), of which, until recently, GUN5 was the only gene to encode a protein of known function, the ChlH subunit of Mg-chelatase. GUN4 was recently shown to encode a novel protein of unknown function that appears to directly interact with GUN5 to both enhance its Mg-chelatase activity, as well as confer on it, ATPase activity (unpublished data RL). Interestingly, there is a reduction in chlorophyll accumulation in both gun4 and gun5 plants and gun4gun5 double mutants produce albino plants. These findings suggest that the tetrapyrrole biosynthetic pathway controls transcription of nuclear genes encoding plastid-localized proteins. In an effort to understanding the mechanism of this signaling pathway more clearly, we propose to structurally characterize the novel protein gun4 and also determine the biophysical mechanism behind gun4/gun5 mediated Mg-chelatase activity. This work will provide a template for further investigation of this unique signaling pathway both in vitro and in vivo in a model eukaryote, Arabidopsis thaliana.
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