A comprehensively curated genome-scale two-cell model for the cyanobacterium Anabaena sp. PCC 7120

Abstract

Anabaena sp. PCC 7120 is a nitrogen-fixing filamentous cyanobacterium. Under nitrogen limiting conditions, a fraction of the vegetative cells in each filament terminally differentiate to non-growing heterocysts. Heterocysts are metabolically and s tructurally specialized to enable O2 -sensitive nitrogen fixation. The functionality of the filament , as an association of vegetative cells and heterocysts, is postulated to depend on me tabolic exchange of electrons, carbon and fixed nitrogen. In the present work, we compile and evaluate a comprehensive curated stoichiometric model of this two-cell system, with the objective function based on the growth of the filament under diazotrophic conditions. The predi cted growth rate under nitrogen replete and deplete conditions, as well as the effect of external carbon and nitrogen sources, was thereafter verified. Furthermore, the model was utilized to comprehensively evaluate the optimality of putative metabolic exchange reaction s between heterocysts and vegetative cells. The model suggested that optimal growth requi res at least four exchange metabolites. Several combinations of exchange metabolites resul ted in predicted growth rates that are higher than growth rates achieved by only considering exchange of metabolites previously suggested in the literature. The curated model of the metabolic network of Anabaena sp. PCC 7120 enhances our ability to understand the metabolic organization of multi-cellular cyanobacteria and provides a platform for further study and engineering of their metabolism