A Comprehensively Curated Genome-Scale Two-Cell Model for the Heterocystous Cyanobacterium Anabaena sp. PCC 7120

Abstract

ORCID IDs: 0000-0002-4155-2085 (D.M.); 0000-0001-7618-8204 (P.R.J.).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 nongrowing heterocysts. Heterocysts are metabolically and structurally specialized to enable O 2 -sensitive nitrogen fixation. The functionality of the filament, as an association of vegetative cells and heterocysts, is postulated to depend on metabolic exchange of electrons, carbon, and fixed nitrogen. In this study, 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 predicted 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 reactions between heterocysts and vegetative cells. The model suggested that optimal growth requires at least four exchange metabolites. Several combinations of exchange metabolites resulted 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 multicellular cyanobacteria and provides a platform for further study and engineering of their metabolism.

Reconstruction of a regulated two-cell metabolic model to study biohydrogen production in a diazotrophic cyanobacterium Anabaena variabilis ATCC 29413

“…Lower bound of exchange reactions for H + , H 2 O, K, SO 4 , Pi, and inorganic ions was set to -1000 mmol/gDCW/h and lower bound of exchange reactions for HCO 3 , NO 3 , photon was set to -10 mmol/gDCW/h. Maximum uptake rate of each carbon source was determined according to the method presented in [36].…”

“…The biomass reaction was used as the objective function to be maximized using FBA and the single-cell model was compared with the metabolic model of Anabaena sp. PCC 7120 [36] for prediction of growth on different carbon sources. So, according to the method presented in [36], mixotrophic growth rate for each carbon source was calculated and divided by autotrophic growth rate on bicarbonate.…”

“…PCC 7120 [36] for prediction of growth on different carbon sources. So, according to the method presented in [36], mixotrophic growth rate for each carbon source was calculated and divided by autotrophic growth rate on bicarbonate. Correlation between the predicted relative growth rates and experimental values for the two models was determined using the Pearson correlation coefficient.…”

“…Two-cell model. The two-cell model was constructed to compare its growth predictions under autotrophic and diazotrophic conditions with those predicted by the regulated two-cell model and predicted by the previous two-cell model [36]. Upper and lower bounds of all intracellular reactions for the heterocyst and vegetative metabolic models were the same as those for the single-cell model, except those presented in Table A in S4 File.…”

“…PCC 6803 [32][33][34][35]. Recently, Malatinszky et al [36] reconstructed a genome-scale metabolic model for the cyanobacterium Anabaena sp. PCC 7120, a heterocyst forming and diazotrophic cyanobacterium.…”