FOCAL predicted glucose/xylose co-utilization conditions. (a) Maximum FBA predicted anaerobic growth of the FOCAL designed E. coli strain as a function of the xylose fraction of the carbon source. The ratio of glucose and xylose within the minimal media was varied while maintaining a constant carbon uptake into the iJO1366 network (110 mmol C·gDW-1·h-1). Under FOCAL's proposed conditions, the strain is incapable of growth when the media is composed entirely of glucose or xylose due to an inability to produce all biomass components. For comparison, the maximum predicted wild-type rate growth is 0.423 h-1 on pure glucose and 0.362 h-1 for pure xylose (not shown). (b) Possible fluxes through central metabolism in the mutant when grown only on glucose. Under these circumstances, the mutant is unable to produce dihydroxyacetone phosphate and glycerol-3-phosphate, which are critical for synthesizing NAD(P) and phospholipids. On xylose only (not shown), the mutant is incapable of sustaining flux beyond the pentose phosphate pathway. Boxed metabolites indicate biomass precursors and dashed arrows indicate multiple reaction steps. Metabolite abbreviations used but not provided in the text are: glc-D, D-glucose; g6p, glucose-6-phosphate; 6pgc, 6-phospho-gluconate; ru5p-D, D-ribulose 5-phosphate; r5p, ribose-5-phosphate; e4p, erythrose-4-phosphate; f6p, fructose-6-phosphate; fru, D-fructose; g3p, glyceraldehyde 3-phosphate; 13dpg, 3-phospho-D-glycerol phosphate; 3pg, 3-phospho-glycerate; 2pg, 2-phospho-glycerate; pep, phosphoenolpyruvate; pyr, pyruvate; accoa, acetyl-CoA.