Pyrimidine biosynthesis in PseudomonasJIuorescens strain A126 was investigated. In this study, de nouo pyrimidine biosynthetic pathway mutant strains were isolated using both conventional mutagenesis and transposon mutagenesis. The resulting mutant strains were deficient for either aspartate transcarbamoylase, dihydroorotase or orotate phosphoribosyltransferase activity. Uracil, uridine or cytosine could support the growth of every mutant strain selected. In addition, the aspartate transcarbamoylase mutant strains could utilize orotic acid to sustain their growth while the orotidine-5'-monophosphate decarboxylase mutant strains grew slowly upon uridine 5'-monophosphate. The wild-type strain and the mutant strains were used to study possible regulation of de nouo pyrimidine biosynthesis in P. JIuorescens. Dihydroorotase specific activity more than doubled after the wild-type cells were grown in orotic acid relative to unsupplemented minimal-medium-grown cells. Starving the mutant strains of pyrimidines also influenced the levels of several de nouo pyrimidine biosynthetic pathway enzyme activities.

Pyrimidine biosynthesis was investigated in Pseudomonas cepacia ATCC 17759. The presence of the de novo pyrimidine biosynthetic pathway enzyme activities was confirmed in this strain. Following transposon mutagenesis of the wild-type cells, a mutant strain deficient for orotidine 5'-monophosphate decarboxylase activity (pyrF) was isolated. Uracil, cytosine or uridine supported the growth of this mutant. Uracil addition to minimal medium cultures of the wild-type strain diminished the levels of the de novo pyrimidine biosynthetic enzyme activities, while pyrimidine limitation of the mutant cells increased those de novo enzyme activities measured. It was concluded that regulation of pyrimidine biosynthesis at the level of enzyme synthesis in P. cepacia was present. Aspartate transcarbamoylase activity was found to be regulated in the wild-type cells. Its activity was shown to be controlled in vitro by inorganic pyrophosphate, adenosine 5'-triphosphate and uridine 5'-phosphate.

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The pyrimidine ribonucleosides uridine or cytidine were shown to serve as a source of nitrogen or carbon for the growth of Pseudomonas fluorescens strain A126. After incubation of either pyrimidine ribonucleoside with extracts of this strain, the resultant catabolic products were detected by thin-layer chromatography. It was found that pyrimidine ribonucleoside catabolism in this pseudomonad involved the enzymes nucleoside hydrolase and cytosine deaminase. The specific activities of both these enzymes could be influenced by the nitrogen or carbon source present in the medium.

Pyrimidine base and nucleoside metabolism in Pseudomonas cepacia was investigated. It was found that this pseudomonad utilized uracil, cytosine or cytidine as a sole nitrogen source while pyrimidine bases and nucleosides supported little bacterial growth as sole carbon sources. Low nucleoside hydrolase and cytosine deaminase activities were detected in P . cepacia. Synthesis of both enzymes appeared to be subject to repression by ammonium ions but repression of cytosine deaminase synthesis was more severe. Synthesis of nucleoside hydrolase or cytosine deaminase was induced if the cells were grown on the substrate cytidine or cytosine, respectively, as a sole nitrogen source.The metabolism of pyrimidine bases and nucleosides has been investigated in few species of the genus Pseudomonas. Unfortunately, pyrimidines were not included in the extensive taxonomic studies concerning the growth of the aerobic pseudomonads upon organic compounds (STANIER et al. 1966, BALLARD et al. 1970).Pyrimidine metabolism has been examined in Pseudomonas acidovorans WARREN 1973, 1974). This pseudoinonad was found to be limited in its utilization of pyrimidine bases and nucleosides due to the absence of a number of salvage pathway enzymes involved in pyrimidine metabolism (KELLN and WARREN 1974). Although pyrimidine biosynthesis has been studied in Pseudomonas aeruginosa and Pseudomonas putida (ISAAC and HOLLOWAY 1968, CONDON et al. 1976), the metabolism of pyrimidines by these microorganisms was not explored with respect to salvage pathways. With relatively little data available about pyrimidine metabolism in pseudomonads, further biochemical characterization of individual species could prove valuable from a taxonomic perspective. The objective of this study was to investigate Pseudomonas cepacia metabolism of pyrimidine bases and nucleosides. This was accomplished by examining its growth on pyrimidine bases and nucleosides as well as by determining which pyrimidine salvage pathway enzymes were present in this pseudomonad.Pseudomonas cepacia ATCC 17759 was utilized in this study (STANIER et al. 1966). The strain was grown in a modified minimal medium of STANIER (1947) that contained 0.1% KH,PO,, 0.1% K,HPO,, 0.1% NaCl, 0.05% sodium citrate -2 H,O, 0.07% MgSO, 7 H,O and 0.4% (NH,),SO,. The medium was adjusted to p H 7.0-7.2 and sterilized. Glucose (0.4%) was added separately as the carbon source. To investigate the ability of P . cepacia to utilize various compounds as sole sources of nitrogen, the strain was grown in liquid minimal medium (5 ml) t h a t lacked (NH,),SO, but contained 0.2% nitrogen source and 0.4% glucose. For sole carbon source determinations, the cells were grown in 5 ml liquid minimal medium (sodinm citrate and glucose omitted) supplemented with 0.2% carbon source and 0.4% (NH,),SO,. Saline-washed cells (approximately lo* cells) were inoculated into liquid medium cultures containing each sole nitrogen or carbon source then the cultures were shaken a t 200 rpm for 8 days a t 30 "C. Optical density a t 600 nm of each...