Abstract

The direct production of ethanol from sweet potato starch by Zymomonas mobilisrequired the construction of four fused glucoamylase genes from Aspergillus awamori using recombinant polymerase chain reaction (PCR) and inserted into the broad-host-range vector pBBR1MCS-2. After electro transformation of the four recombinant plasmids into Z. mobilis, it was discovered that only the plasmid pGA0 offered the transformants the phenotype of growth on rich medium (RM) plates containing 1.5% sweet potato starch. One transformant had the highest glucoamylase activity of 157 U/mL and about 80% of enzyme activities were detected in extra cellular fraction, indicating that the glucoamylase-coding sequence ofAspergillus awamori is expressed as an active enzyme. All transformants of pGA0 could directly ferment sweet potato starch to ethanol. One transformant displayed higher efficiency of ethanol production with 14.73-fold of control strain and 92.69% of the theoretical yield of ethanol. Kinetics of this transformant was also investigated, including growth curve, total sugar consumption, and ethanol production. Our results provide a basis for further constructing a genetically engineered Z. mobilis strain directly fermenting sweet potato starch with higher efficiency.

Key words: Ethanol, sweet potato starch, Zymomonas mobilis, Aspergillus awamori,glucoamylase.