High-level secretion of a chimeric thermostable lichenase from Bacillus subtilis by screening of site-mutated signal peptides with structural alterations

A chimeric gene mHG (669 bp) was constructed by substitution of Clostridium thermocellum ZJL4 lichenase (CG) N-terminal fragment (except its signal sequence) for the counterpart of Bacillus sp. A3 lichenase (BG). To acquire high-level secretion of the chimeric lichenase (mHG) in Bacillus subtilis, a series of site-mutated signal peptides were designed. The activity of mHG, which was directed by an artificial hydrophobic signal peptide H1 (MMARKIAGMATSLLVIFSSSAVA) from cytoplasm into growth medium, reached 80.56 U/ml after 22-h incubation, indicating that signal peptide hydrophobicity appears to be critical for early stages in mHG export. By purification of the mHG (similar to 25.3 kDa) from cultures of B. subtilis (pBSG- H1), enzymatic property assays showed that the common optima for mHG were 70 degrees C and pH 5.0. The residual activity of mHG at 90 degrees C for 10 min was 83.45% of its maximum activity, which was almost similar to that of CG (90 degrees C, 10 min, 84.33%). This constructed shuttle expression vector with a novel signal peptide exhibited its applicability for high-level production of heterologous proteins from B. subtilis. Moreover, the high-level secreted mHG with relatively high thermostability could be a potential candidate for feed industrial applications.