1. The mouse Gardner lymphoma 6C3HED was grown in ascites fluid in a form sensitive to the action of l-asparaginase (line 1), in another form which was resistant to l-asparaginase (line 2) and in a third form with partial sensitivity to l-asparaginase (line 3).

2. The l-asparaginyl-tRNA synthetase activities of extracts of the tumour cells, cultured both in the mouse and in vitro, were determined. Two of the lines, 1 and 3, in early passage numbers, showed a derepression mechanism involving l-asparagine. Mutation occurred with these lines resulting in the l-asparaginyl-tRNA synthetase activity of all the tumour cell lines being the same.

3. Cells of line 1 had low l-asparagine synthetase activity, which was unchanged by altering the supply of l-asparagine in vitro. Cells of lines 2 and 3 exhibited l-asparagine synthetase activities, which changed with the supply of l-asparagine.

4. It is not certain that l-asparagine synthetase activity of l-asparaginase-sensitive cells is controlled by l-asparaginyl-tRNA acting as a corepressor.

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