Amino Acid Substitution of Thermostability Related Sites by PCR

The three thermostability-related sites of the DCase were substituted (15) through site-directed random amino acid changes by use of the PCR technique according to Ito et al. (20), and then thermostabilized DCase-producing strains were screened by means of the colorimetric colony assay method. As a result of DNA analysis of the mutant thermostabilized DCases, it was proved that the following amino acid changes at the sites increased the thermosta-bility in addition to the known amino acid changes; the 57th amino acid of DCase was changed to leucine; the 203rd amino acid to alanine, asparagine, glutamate, his-

tidine, isoleucine, or threonine; and the 236th amino acid to serine or threonine. These results are shown in Table 4.

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