Cloning and expression of the N-acetylmuramidase gene from Streptomyces rutgersensis H-46

The N-acetylmuramidase SR1 gene from Streptomyces rutgersensis H-46 was cloned in Escherichia coli JM109 and expressed in E. coli BL21(DE3)pLysS. An open reading frame included the leader peptide region encoding a polypeptide of 65 amino acid residues and the mature SR1 enzyme region encoding a polypeptide of 209 amino acid residues. The overall G + C content of the mature enzyme gene was 67.6%, with 98.1% of G or C in the third position of the codons. The calculated molecular weight of the mature enzyme was 23,057 Da. The amino acid sequence of the mature enzyme showed a significant level of identity with bacteriolytic enzymes from Streptomyces globisporus (50.9% identity), Chalaropsis species (40.2% identity) and Saccharopolyspora erythraea (31.0% identity). The mature enzyme gene cloned into plasmid pET26b carrying a signal peptide, peIB, was expressed in E. coli BL21(DE3)pLysS. The signal peptide region was cleaved during the production of the enzyme. Specific activity of the enzyme purified from the transformant was almost identical to that of the native enzyme. Furthermore, the SR1 enzyme gene cloned with the leader peptide gene into plasmid pET28a was also expressed in E. coli. In this case, a proform-like protein was partially processed; 35 amino acid residues were cleaved but 30 amino acid residues remained. This proform like protein has approximately one-nineteenth the activity of the native enzyme. These results indicated that the native SR1 enzyme was produced in the following manner in the cells of S. rutgersensis H-46. The SR1 enzyme gene was translated to a pre-proform protein followed by the deletion of a signal peptide. Finally, the proform-like protein was processed by deletion of the remaining leader peptide.