title: Analysis of polyhydroxybutyrate flux limitations by systematic genetic and metabolic perturbations.
abstract: Poly-3-hydroxybutyrate (PHB) titers in Escherichia coli have benefited from 10+ years of metabolic engineering. In the majority of studies, PHB content, expressed as percent PHB (dry cell weight), is increased, although this increase can be explained by decreases in growth rate or increases in PHB flux. In this study, growth rate and PHB flux were quantified directly in response to systematic manipulation of (1) gene expression in the product-forming pathway and (2) growth rates in a nitrogen-limited chemostat. Gene expression manipulation revealed acetoacetyl-CoA reductase (phaB) limits flux to PHB, although overexpression of the entire pathway pushed the flux even higher. These increases in PHB flux are accompanied by decreases in growth rate, which can be explained by carbon diversion, rather than toxic effects of the PHB pathway. In chemostats, PHB flux was insensitive to growth rate. These results imply that PHB flux is primarily controlled by the expression levels of the product forming pathway and not by the availability of precursors. These results confirm prior in vitro measurements and metabolic models and show expression level is a major affecter of PHB flux.
authors.affiliation: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: Keith E J
authors.initials: KE
authors.lastname: Tyo
authors.forename: Curt R
authors.initials: CR
authors.lastname: Fischer
authors.forename: Fritz
authors.initials: F
authors.lastname: Simeon
authors.forename: Gregory
authors.initials: G
authors.lastname: Stephanopoulos
mesh_terms: D020155:3-Hydroxybutyric Acid / Q000378:metabolism; D000429:Alcohol Oxidoreductases; D001669:Biochemical Phenomena; D002244:Carbon / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism*; D015870:Gene Expression; D006885:Hydroxybutyrates; D008954:Models, Biological; D055585:Physical Phenomena; D011091:Polyesters; D012106:Research; D000094122:Polyhydroxybutyrates
chemical_list: D020155:3-Hydroxybutyric Acid; D000429:Alcohol Oxidoreductases; D002244:Carbon; D006885:Hydroxybutyrates; D011091:Polyesters; C003182:poly-beta-hydroxybutyrate; D000094122:Polyhydroxybutyrates; C023744:acetoacetyl-CoA reductase