in conjunction with bleach, which is a strong oxidizing agent. When used with bleach, the methionine in subtilisin is oxidized and the enzyme is inactivated, preventing the subtilisin from doing its work of breaking down the proteins present in food stains, blood stains, and the like.

To overcome this problem, genetic engineering techniques were used to isolate the gene for subtilisin, and the small part of the gene that codes for methionine 222 was replaced by chemically synthesized DNA fragments that coded for other amino acids. The experiment was done in such a way that nineteen new subtilisin genes were produced, and every possible amino acid was tried at position 222. Some of the altered genes gave rise to inactive versions of the enzyme, but others resulted in fully functional subtilisin. When these subtilisins were tested for their resistance to oxidation, most were found to be very good (except when cysteine replaced methionine: It too is easily oxidized). So now it is possible to use laundry detergent and bleach at the same time and still remove protein-based stains. This type of gene manipulation, which has been called "protein engineering," has already been used for making beneficial changes in other industrial enzymes, and in proteins used for medical purposes.

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