[Improving soluble expression of Geobacillus sp. B1 CGTase by errorprone PCR]

This study was aimed to enhance the extracellular enzymes activities and soluble expression of CGTase from Geobacillus sp. B1 by directed evolution.

A library of CGTase mutants was constructed by introducing random mutagenesis using error-prone PCR to screen mutant enzymes with improved extracellular enzyme activities and soluble expression. After induction, expression and purification, the mutant enzyme was characterized.

After screening, two optimum mutants ds-6 and ep-9 with extracellular alpha-cyclization activity are respectively 1.72 times and 2.18 times of the original enzyme. The sequence of ep-9 cgt gene showed that three nucleotides substitution, G2005A, A2037G and T2081G were observed, and two of them caused amino acid changes. According to the 3D structure of Geobacillus sp. B1 cyclodextrin glucosetransferase mimicked by SWISS-MODEL Repository, two amino acid mutations were in rotating angle between beta angle and the random coil. The wild-type CGTase or ep-9 genes was ligated with pET-28 (a)-OmpA vector, and expressed in E. coli BL21 (DE3). After induced by lactose, CGTases were purified and characterized. The results showed that the specific β-cyclization activity of the evolved CGTase was 1.31-fold than that of the wild-type CGTase, and the Km decreased from 4.3 to 3.74 g/L. The pH stability of the evolved CGTase was better than wild-type CGTase. Site-directed mutagenesis demonstrated the key to improve the soluble expression level and extracellular enzyme activity was G2005A.

Directed evolution by error-prone PCR of Geobacillus sp. B1 CGTase gene is effective to improve extracellular enzyme activities and soluble expression, in particular mutation occurred in the G2005A.