Growth inhibition by NH 4 + ions in arginine-requiring mutants of Chlamydomonas reinhardi

Summary In Chlamydomonas reinhardi, two types of arginine-requiring mutants can be distinguished: one type grows on media containing the usual concentration of NH₄Cl (400 mg/l); the other type can only grow on media poor in NH₄⁺ ions (NH₄⁺-sensitive mutants). The causes of NH₄⁺-sensitivity have been investigated. The results strongly support the view according to which NH₄⁺-sensitivity is a potential property of the wild-type strain, which becomes effective when a block is introduced in the biosynthetic pathway of arginine. Accordingly, two events are always necessary for obtaining an NH₄⁺-insensitive argininerequiring mutant, the first one being responsible for the nutritional requirement, the second one leading to reduced sensitivity to ammonium. This explains why arginine-requiring mutants preferentially arise on selective media deprived of ammonium ions.Chargé de Recherches du Fonds National de la Recherche Scientifique