Control of carbon flux to glutamate excretion in Klebsiella pneumoniae: the role of the indigenous plasmid and its encoded isocitrate dehydrogenase
Klebsiella pneumoniae (NCTC, CL687/80) harbors a large indigenous plasmid (pC3), which in addition to encoding for citrate utilization, proline synthesis and glutamate excretion, it uniquely carries the structural gene (icd); encoding isocitrate dehydrogenase (ICDH). Flux analysis revealed that ICDH, despite its role in the generation of NADPH required for glutamate dehydrogenase, is not rate-limiting (controlling) in central metabolism as evidenced by a negative flux control coefficient and an adverse effect of overexpression (14-fold) on glutamate excretion. More significantly, however, this paper presents, for the first time, clear evidence that the accumulation of glutamate and its subsequent excretion is associated with the C3 plasmid-encoded regulatory elements, which trigger a shift-down in the activity of α-ketoglutarate dehydrogenase, both in the K. pneumoniae parental strain as well as in the E. coli exconjugants strains. This finding opens the door for the exploitation of regulatory elements as a tool for manipulating flux in microbial cell factories.
KeywordsKlebsiella pneumoniae Glutamate excretion Isocitrate dehydrogenase Glutamate dehydrogenase α-Ketoglutarate dehydrogenase
MEM wishes to thank Chief Michael Ade.Ojo for his entrepreneurial vision and moral values and Professor Valentine A. Aletor, VC, Elizade University, for constant interest in research and encouragements. The authors wish to thank Adrian Agho and Adesanya Adetayo for skilled assistance in the preparation of the graphs. FT and AMC wish to acknowledge the James G. Martin Genomics programme for technical and logistical support to FT.
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