Nineteen chlorate-resistant variants were isolated after mutagenesis from cells of Medicago coerulea. The level of nitrate reductase activity was variable in these lines and ranged from 100% to less than 5% of the wild type level. Xanthine dehydrogenase was not affected in any of those variants tested. Methylammonium-resistant variants were also isolated from the same type of cells. They show a different regulation of nitrogen utilization. In particular, the enzymatic level of nitrate reductase which, in wild type cells, is sensitive to ammonium repression, is much less affected in the variants. Differences were also seen in the regulation of other functions of the nitrogen-utilizing pathway: xanthine dehydrogenase and, possibly, adenine uptake. © 1986 Springer-Verlag.

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