## 6 Genome-Scale Modeling: A Suitable Platform for Improving CA Production
clawuligerus_ (containing 100 reactions and 91 metabolites) expanded from the first model was used to explore the effect of nutrients (carbon, nitrogen, and phosphate) and oxygen limitations on CA production [44; 167]. The authors found that maximization of ATP yield was the best objective function as it was the most suitable function for predicting CA production. Likewise, they found that during the phosphate limitation, CA production was favored when glutamate flux and the precursors of the urea cycle increased. In 2018, two model updates of iMM864 were published. First, the iLT1021 GEM containing 1494 reactions, 1360 metabolites, and 1021 genes, using ATP yield as the best objective function to represent _S. clawuligerus_ cultivated in chemostat mode, while growth rate was used as the objective function for the batch cultures [165]. The authors identified the NAD+ synthase, _glyK_, and _idh_ genes as the best metabolic targets for improving the CA production. In 2018, [PERSON] and colleagues [43] also improved the iMM864 GEM  
Figure 4: The evolution of _S. clawuligerus_ metabolic models. Species abbreviations are Sclav: _Streptomyces clawuligerus_, _Sco: Streptomyces coelicolor_, _See: _Saccharomyces cerevisiae_, _Eco: Escherichia coli_. Similarly, number of genes, metabolites, and reactions were included after a/symbol, respectively. by including gene-reaction associations and gap-filling curation. This model was used to characterize the TCA strategies used by _S. clavuligerus_ at four different growth conditions associated with CA production [43]. The authors found that CA biosynthesis correlates positively with accumulation of oxaloacetate, succinate, and acetate, while it correlates negatively with accumulation of malate at different feeding conditions in continuous cultures [43]. Additionally, FBA-fluxes inferred from experimental data predicted that the phosphoenolpyruvate carboxylase activation triggers CA production. In contrast, the activation of the glyoxylate shunt (by isocitrate lyase) matched with carbon limitation conditions and a negative effect on CA titers [43]. The last model was updated and curated in 2019, containing 1534 reactions, 1199 metabolites and 871 genes [166]. Although this model was not validated with experimental data, it explored 18 objective functions and metabolic phenotypes of _S. clavuligerus_ during production of CA and Cephamycin C.  
Finally, in 2020, a new and independent reconstruction was published [19]. The topologically and thermodynamically curated iDG1237 GEM encompasses 2177 reactions, 1518 metabolites, and 1237 genes. This model was validated with experimental data and used to get insights about the metabolic reprogramming strategies of _S. clavuligerus_ under high and low shear stress conditions (intracellular nutrient availability). The authors showed that during phosphate-limited and low shear stress conditions the CA production halved due to the existence of overflow metabolism coincident with accumulation of TCA intermediates. These evidences suggest that GEMs constitute valuable platforms for exploring the metabolism of _S. clavuligerus_ under different experimental and nutritional conditions. We invite the _S. clavuligerus_ community to use these models along with multi-omics datasets for getting further insights about the regulatory links between genes, proteins and metabolism in order to improve CA production at the industrial level.