Figure 6. Dysregulation of the ICL/ICD rheostat balance impairs growth on acetate.

(a) Growth of wild-type, attB::P_npicd1 and attB::P_hspicd1 strains of M. bovis BCG in glucose-based (left) or acetate-based (right) minimal medium without glutamate. The attB::P_npicd1 strain expresses icd1 from the native promoter. The attB::P_hspicd1 strain overexpresses icd1 from the strong hsp60 promoter. Data are means±s.d. (n=3 independent experiments). (b) Computationally modelled phase planes of carbon flux between ICD and ICL in glucose-based (left) or acetate-based (right) medium. Heat maps show scales of predicted growth (maroon, best growth; blue, zero growth) at varying levels of ICD/ICL flux. The dynamic range of ICD flux allowing optimal growth at each ICL flux value is narrower in acetate-grown cells compared with glucose-grown cells. (c) Bifurcation of metabolic fluxes between the oxidative TCA cycle and the glyoxylate shunt in E. coli and mycobacteria. In E. coli, the substrate K_m of ICL is almost 100-fold higher than the substrate K_m of ICD. Partial inhibition of ICD by AceK-mediated phosphorylation enables partitioning of one-third of the flux into the glyoxylate shunt with the remaining two-thirds of the flux continuing through the oxidative TCA cycle. In mycobacteria, the substrate K_m of ICL1 is only slightly higher than the substrate K_m of ICD2. In this context, glyoxylate-mediated activation of ICD functions as a metabolic rheostat to maintain the optimal partitioning of fluxes between the TCA cycle and the glyoxylate shunt. ICT, isocitrate; GOX, glyoxylate; α-KG, alpha-ketoglutarate.