The intent of the project is to identify the biochemical mechanisms that control the differentiation of vegetative cells into heterocysts (sites of aerobic nitrogen fixation) in Nostoc spp. Dr. Meeks hypothesizes that heterocyst differentiation is under control of a global nitrogen regulatory system in the free- living state, that may be superseded by a symbiotic specific system when Nostoc is in association with the bryophyte anthoceros. He proposes to continue characterization and complementation - to identify and isolate regulatory genes involved in heterocyst differentiation. The specific objectives include isolation of additional mutants with the pleiotropic phenotype of inability to grow with nitrate and N2 as nitrogen sources, but able to fix N2 in symbiosis. Such mutants will be characterized by analysis of classes of revertant (single site suppressor mutations). The mutant phenotypes will be complemented by conjugational transfer of wild type or "revertant" DNA cloned in E. coli or packaged in vitro in plasmids and transferred into the Nostoc recipient by electroporation. complementing fragments will be sequenced and compared to established data banks. They will also be used in physiological genetic studies with wild type and mutant strains of Nostoc to define their role in differentiation. %%% This study has implications for two areas. First it is a rare example of pattern formation in bacteria and therefore provides an example to study with the powerful genetic techniques available in bacterial system. Additionally, the cyanobacteria are important components of the environment and play a unique role as primary producers of reduced carbon and nitrogen. An understanding of how these processes are controlled is therefore of general importance.
