The nature of the reserves of Pseudomonas aeruginosa that are oxidized during endogenous respiration was studied by following the changes in the chemical constituents and in the distribution of radioactivity of starving cells that had been grown on C¹⁴-labeled substrates. The total protein and nucleic acid of cell suspensions decreased during starvation. Deoxyribonucleic acid increased slightly, whereas ribonucleic acid decreased. C¹⁴O₂ was evolved from endogenously respiring cells specifically labeled in the nucleic acid fraction and from cells specifically labeled in the protein fraction. Chemical fractionation of C¹⁴-labeled cells showed a decrease in hot trichloroacetic acid-soluble and insoluble compounds, indicating that the C¹⁴O₂ arose from the degradation of RNA and protein and not from free pool compounds. A decrease in ribosomal RNA and protein was evident from physical fractionations of starved labeled cells. An enzyme responsible for the initiation of ribosomal degradation was found to be associated with the ribosome fraction and was identified as polynucleotide phosphorylase. The enzyme was inactive in high magnesium concentrations but was active under conditions which allowed the dissociation of the large ribosomal units into 50S and 30S components. Polynucleotide phosphorylase was not solubilized by the dissociation of the 70S ribosomes but remained firmly attached to the 50S subunit. The oxidation of exogenous substrates resulted in varying degrees of suppression of the oxidation of endogenous RNA and this suppression was attributed to the relative stabilizing effect that the exogenous substrates exerted on the ribosomes. The oxidation of endogenous protein was depressed during the oxidation of exogenous glucose, aspartic acid and adenosine and was increased during the oxidation of α-ketoglutaric acid. The response of endogenous respiration to the oxidation of exogenous substrates appeared to be related to a requirement for ammonium ions for assimilation of carbon. / Land and Food Systems, Faculty of / Graduate