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Modification of the cysteine metabolism pathway

Sulphur rich amino acids (cysteine and methionine) as well as lysine are amino acids, which are not produced by monogastric animals and need therefore to be supplied by their diet. Corn seeds, which are extensively, used as animal feed, have a limited amount in those sulphur rich amino acids. Cysteine synthesis involves production of the carbone backbone from serine through the enzyme serine acetyltransferase (SAT), and addition of sulphur, which is provided through the action of the enzyme adenosyl phosphosulfate reductase (APR). As a mean to over-produce cysteine production in maize, a gene encoding Adenosine 5' Phosphosulfate Reductase from Lemna minor, has been introduced into maize plants.

Serine acetyl transferase (SAT) is a key enzyme in the cysteine biochemical pathway in activating L-serine by transfer of an acetyl group to generate O-acetyl-serine, an intermediate in cysteine production. A bacterial, cysteine insensitive SAT has been cloned behind a constitutively expressed promoter and introduced into maize. The over-produced cysteine could then be trapped into sulphur-rich proteins, i.e. the sunflower 2S seed albumin (SSA) protein. Expression of the SSA alone did not lead to increases in total cysteine and methionine levels, probably due to a limitation in the cysteine, methionine synthesis. Therefore an approach combining both the SAT enzyme and the SSA protein has been undertaken.

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