To elevate the production of foreign target proteins using a heterologous protein secretion system of Bacillus subtilis, two different pleiotrophic mutations, degUh for increase of transcriptional level of target genes and spoOA for reduction of extracellular protease activity of a host strain were introduced, respectively. The productivities of three differently originated enzymes, beta-lactamase, streptokinase and human pancreatic trypsin inhibitor (hPSTI) were examined under the each mutation background. By the degUh mutation, the activities of all three enzymes secreted in the culture were increased, although the increased levels were different from 1.2-fold (streptokinase) to 1.8-fold (beta-lactamase). The lower productivity of streptokinase compared to other enzymes under the degUh background was caused by the higher susceptibility to proteolytic degradation. The increased transcriptional level of the beta-lactamase gene by degUh mutation resulted in the accumulation of unprocessed precursor protein in the cytoplasm and cytoplasmic membrane. In the case of the spoOA mutation background, the differences in the levels of the secreted target proteins were not significant and observed only after a stationary phase of the growth.
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