Control of Cellular Differentiation in Symbiotically Competent Cyanobacteria

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*** 9514787 Meeks The long-term goal of this program is to identify, characterize and manipulate genes involved in cellular differentiation in the cyanobacterium Nostoc sp. strain ATCC 29133, a strain that can differentiate motile hormogonial filaments, akinetes (spores) and heterocysts, which are the sites of N2 fixation. Nostoc 29133 is also a facultative heterotroph that can establish a symbiotic association, via hormogonia, with the bryophyte Anthoceros punctatus. Anthoceros produces extracellular metabolites that sequentially stimulate hormogonia and heterocyst differentiation of Nostoc spp. to 10-fold or higher levels than those seen in its absence. The physiological and molecular genetic techniques developed in the prior grant and the transposon mutants currently in hand form the methodological and biological foundation of this proposal. Specific objectives are proposed in the following two areas. I . Heterocyst maturation is dependent on a signal cascade regulatory system. Analysis of Nostoc 29133: :Tn5-1063 strain UCD 311 (Fox~Sym+) with an insertion devR, encoding a receiver-domain-only response regulator protein. Biochemical and genetic assays to identify potential phosphodonors and phosphoreceivers of DevR. This project will identify members of a two component regulatory system involved in heterocyst development. 2. Hormogonia differentiation is a consequence of differential gene expression. Establishment and analysis of a hormogonia-specific subtractive cDNA library; generation by transposition of mutants of Nostoc 29133 altered in hormogonia formation; genetic characterization of such mutants and of cosmic clones from cDNA analysis. This project is the initial effort in understanding how cells divide uncoupled from DNA replication and biomass increase to form hormogonia and then return to the vegetative growth state in completing a cellular life cycle. ***