Recent research has identified several large families of prokaryotic global regulators. Results from the PI's laboratory have shown that members of the LysR, two-component, and MerR families of transcriptional regulatory proteins regulate Bradyrhizobium japonicum nod gene expression. Regulation of nod gene expression in B. japonicum may represent an interesting and useful model system in which to explore the interactions between these global regulatory systems. The focus of this project is the MerR orthologue, NolA. Recent results show that this protein is a central player in the repression of nod gene expression in response to a variety of novel signal molecules. NolA mediates its repressive effects by induction of NodD2, which represses nod gene expression. Results from the PI's laboratory indicate that NolA expression is responsive to the level of intracellular, tetrameric Nod signal produced in B. japonicum. Therefore, NolA is a component of a unique feedback, regulatory loop that controls nod gene expression. Nol1A expression is induced by a cell density factor (CDF, a glycosylated flavonoid) and a plant-produced signal that is destroyed by chitinase treatment. The objectives of this research project are to 1. confirm the chemical structure of the CDF, explore its activity, and survey other bacteria for the presence of such a novel, quorum sensing molecule; 2. identify other genes involved in NolA action using a combination of genomic and mutagenic approaches and 3. isolate and chemically characterize the chitin-like plant inducer of nolA expression. These studies should clarify the role of NolA in nod gene expression. The results obtained, when added to other on-going research, should give a more complete picture of nod gene expression in B. japonicum and may serve as a paradigm for other complex regulatory systems where global regulatory networks impinge on the expression of a single operon. The B. japonicum-soybean symbiosis is an important model system for exploring the cell signaling pathways between host and symbiont. The ultimate goal of this work is a clearer understanding of the integration of regulatory mechanisms involved in the global regulation of cellular metabolism.
