Engineering and metabolic characteristics of a Clostridium tyrobutyricum strain / 生物工程学报

Clostridium tyrobutyricum is suitable for simultaneous saccharification and fermentation of lignocellulosic. It can produce butyric acid, acetic acid as its main fermentation products from a wide variety of carbohydrates such as glucose, xylose, cellobiose and arabinose. In order to decrease acetic acid content and increase butyric acid content in C. tyrobutyricum, we replaced genes on the acetic acid fermentation pathway with genes on the butyric acid fermentation pathway. Three genes were selected. They were acetyl-CoA acetylrtansfers gene (thl) which is the key enzyme gene associated with acetic acid fermentation pathway from Clostridium acetobutylicum, erythromycin gene (em) from plasmid pIMP1 and phosphotransacetylase gene (pta) which is the key enzyme gene associated with butyric acid fermentation pathway from C. tyrobutyricum. We fused these genes with pUC19 to construct nonreplicative integrated plasmids pUC19-EPT. Then we transformed pUC19-EPT into C. tyrobutyricum through electroporation. The recombinant transformants grown on plates containing erythromycin were validated by PCR. A mutant whose pta gene was displaced by thl gene on the chromosome was selected. In the fermentation from glucose, the mutant's yield of butyric acid is 0.47, increased by 34% compared with wild type; and the yield of acetic acid is 0.05, decreased by 29% compared with wild type.

Breeding of Actinobacillus succiniogenes mutants with improved succinate production based on metabolic flux analysis / 生物工程学报

It is very important to obtain high yield mutant strains on the base of metabolic flux analysis of Actinobacillus succinogenes S.JST for the industrial bioconversion of succinic acid. The metabolic pathway was analized at first and the flux of the metabolic networks was calculated by matrix. In order to decrease acetic acid flux, the strains mutated by soft X-ray of synchronous radiation were screened on the plates with high concentration of fluoroacetic acid. For decreasing the metabolic flux of ethanol the site-directed mutagenesis was carried out for the reduction of alcohol dehydrogenase(Adh) specific activity. Then the enzyme activity determination and the gene sequence analysis of the mutant strain was compared with those of the parent strain. Metabolic flux analysis of the parent strain indicated that the flux of succinic acid was 1.78(mmol/g/h) and that the flux of acetic acid and ethanol were 0.60 (mmol/g/h) and 1.04( mmol/g/h), respectively. Meanwhile the metabolic pathway analysis showed that the ethanol metabolism enhanced the lacking of H electron donor during the synthesis of succinic acid and that the succinic acid flux was weakened by the metabolism of byproducts ethanol and acetic acid. Compared with the parent strain, the acetic acid flux of anti-fluoroacetic mutant strain S.JST1 was 0.024 (mmol/g/h), decreasing by 96%. Then the enzyme determination showed that the specific activity unit of phosphotransacetylase(Pta) decreased from 602 to 74 and a mutated site was founded in the pta gene of the mutant strain S.JST1. Compared with that of the parent strain S.JST1 the ethanol flux of adh-site-directed mutant strain S.JST2 was 0.020 (mmol/g/h), decreasing by 98%. Then the enzyme determination showed that the specific activity unit of Adh decreased from 585 to 62 and the yield of end product succinic acid was 65.7 (g/L). The interdiction of Adh and Pta decreased the metabolism of byproducts and the H electron donor was well balanced, thus the succinic acid flux was strengthened by the redundant carbon flux from these byproducts. The mutant strain S.JST2 obtained in this paper deserves being extended to application of industrial fermentation.

Biosynthesis of poly (3-mercaptopropionate) and poly (3-mercaptopropionate-co-3-hydroxybutyrate) with recombinant Escherichia coli / 生物工程学报

Polythioesters newly emerged as a type of novel polymer and they have showed great potential for application in industries. In this study, genes of butyrate kinase (buk) and phosphotransbutyrylase (ptb) from Clostridium acetobutylicum, and poly (3-hydroxybutyrate) (PHB) synthase gene from Thiocapsa pfennigii were used for construction of a metabolic pathway to synthesize the polythioesters. When 3-mercaptopropionate and 3-hydroxybutyrate were fed, poly (3-mercaptopropoinate) [poly (3MP)] and poly(3-mercaptopropionate-co-3-hydroxybutyrate) [poly(3MP-co-3HB)] were synthesized by recombinant Escherichia coli JM109 (pBPP1) harboring the constructed metabolic pathway. Results indicated clearly that all these genes are necessary for the synthesis of poly(3MP) and poly(3MP-co-3HB).