High-Throughput Screening of T7 Promoter Mutants for Soluble Expression of Cephalosporin C Acylase in E. coli

Cephalosporin C acylase (CCA) is the key enzyme in the production of 7-aminocephalosporanic acid (7-ACA) via a one-step enzymatic process. To improve the soluble expression level of CCA in recombinant Escherichia coli at elevated temperatures, a library of T7 promoter mutants was created by site-saturation mutagenesis, and a series of mutated promoters were subsequently screened. Green fluorescent protein (GFP) was fused to the C-terminus of CCA to facilitate library screening, and the expression of the CCA and GFP fusion proteins was investigated under the control of the T7 promoter. Twenty-four mutants were selected by detecting the fluorescence intensity of colonies on agar plates to form a library with different expression levels. The enzyme activities of the mutants were positively correlated with their fluorescence intensities. The highest enzyme activity among these mutant promoters was 1.3-fold higher than the enzyme activity resulting from the wild-type promoter when the cells were cultured at 32 °C for 16 h. In addition, the transcription and expression levels of several typical promoters were discussed, and the effects of GFP fusion on the enzyme activity of CCA were investigated.