The pathway for the utilization of L-arginine, agmatine, L-ornithine, and putrescine as the sole nitrogen source by Escherichia coli K-12 has been elucidated. Mutants impaired in the utilization of one or more of the above compounds were isolated, and their growth on the different compounds as a sole source of nitrogen and the activities of enzymes of the putative pathway were examined. Our results show that L-arginine is first decarboxylated to agmatine, which is hydrolyzed to urea and putrescine. L-Ornithine is decarboxylated to putrescine. Putrescine is transaminated to y-aminobutyraldehyde, which is oxidized to y-aminobutyric acid.-y-Aminobutyric acid is degraded to succinate.

The regulation of the synthesis of the enzymes involved in the utilization of L-arginine, L-ornithine, agmatine, and putrescine as a sole nitrogen source in Escherichia coli K-12 was examined. The synthesis of agmatine ueohydrolase, putrescine aminotransferase, and pyrroline dehydrogenase is dually controlled by catabolite repression and nitrogen availability. Catabolite repression of agmatine ureohydrolase, but not that of putrescine aminotransferase or pyrroline dehydroge!nase, is relieved by the addition of cAMP. Agmatine ureohydrolase synthesis im addition is subject to induction by L-arginine and agmatine. Arginine decarboxylase and ornithine decarboxylase synthesis is not sensitive to catabolite repression or to stimulation by nitrogen limitation or subject to substrate induction.

In the heterothallic myxomycete Physarum polycephalum, uninucleate amoebae normally differentiate into syncytial plasmodia following heterotypic mating. In order t o study the genetic control of this developmental process, mutations affecting the amoebal-plasmodia1 transition have been sought. Numerous mutants characterized by self-fertility have been isolated. The use of alkylating mutagens increases the mutant frequency over the spontaneous level but does not alter thc mutant spectrum. Three spontaneous and 14 induced mutants have been analyzed genetically. In each, the mutation appears to be linked to the mating type locus. In three randomly selected mutants, the nuclear DNA content is the same in amoebae and plasmodia, indicating that amoebal syngamy does not precede plasmodium development in these strains. These results indicate that a highly specific type of mutational event, occurring close to or within the mating type locus, can abolish the requirement for syngamy normally associated with plasmodial differentiation. These mutations help define a genomic region regulating the switch from amoebal to plasmodial growth.

The haploid myxamoebae of Physarum polycephalum reversibly differentiate to form dormant microcysts under conditions of starvation. The thin-walled cysts can be selective recovered from a cell suspension which has been treated with the surfactant Triton X-100 to lyse amoeboid forms. Excystment, which is initiated by suspension in liquid medium, is inhibited by antibiotics which block protein synthesis. Cysts of drug resistant mutants excyst rapidly in media containing sufficient antibiotic to maintain drug sensitive strains in the encysted state. The selective survival of non-excysted cells following Triton X-100 treatment has been employed to enrich for drug sensitive mutants. Several anisomycin sensitive mutants have been isolated, one of which has been analysed genetically. The possible applications of this mutant enrichment technique are discussed.