Utilization of amino acids by Chromatium sp. strain D

Organic nitrogen metabolism of phototrophic bacteria

Recent reviews dealing with phototrophic bacteria are concerned with bioenergetics, nitrogen fixation and hydrogen metabolism, synthesis of the photosynthetic apparatus and phylogeny/taxonomy. The organic N-metabolism of these phylogenetically diverse bacteria has last been reviewed in 1978. However, amino acid utilization and biosynthesis, ammonia assimilation, purine and pyrimidine metabolism and biosynthesis of delta-aminolevulinic acid as precursor of bacteriochlorophylls and hemes are topics of vital importance. This review focuses on utilization of amino acids as N- and C/N-sources, the pathways of purine and pyrimidine degradation, novel aspects of amino acid biosynthesis (with emphasis on branched-chain amino acids and delta-aminolevulinic acid) and some aspects of ammonia assimilation and glutamate synthesis by purple bacteria, green sulfur bacteria and Chloroflexus aurantiacus.

Active transport in phototrophic bacteria

Phototrophic bacteria utilize light-driven, cyclic electron flow to pump protons out of their cytoplasm, creating an electrochemical proton gradient, ΔμH+, outside acid and positive. These bacteria exchange external protons for internal cations (Na(+), K(+) and Ca(+2)), allowing the cells to maintain a nearly constant internal pH while maintaining the electrical component of ΔμH+. Na(+)/H(+) exchange also establishes an electrochemical Na(+) gradient. Phototrophic bacteria are able to utilize these electrochemical gradients as energy sources for the uptake of a wide variety of metabolites (e.g., sugars, organic acids and amino acids) via metabolite/cation symports.

Nitrogen assimilation in Rhodopseudomonas acidophila

Rhodopseudomonas acidophila strain 7050 assimilated ammonia via a constitutive glutamine synthetase/glutamate synthase enzyme system. Glutamine synthetase had a Km for NH+4 of 0.38 mM whilst the nicotinamide adenine dinucleotide linked glutamate synthase had a Km for glutamine of 0.55 mM. R. acidophila utilized only a limited range of amino acids as sole nitrogen sources: L-alanine, glutamine and asparagine. The bacterium did not grow on glutamate as sole nitrogen source and lacked glutamate dehydrogenase. When R. acidophila was grown on L-alanine as the sole nitrogen source in the absence of N2 low levels of a nicotinamide adenine dinucleotide linked L-alanine dehydrogenase were produced. It is concluded, therefore, that this reaction was not a significant route of ammonia assimilation in this bacterium except when glutamine synthetase was inhibited by methionine sulphoximine. In L-alanine grown cells the presence of an active alanine-glyoxylate aminotransferase and, on occasions, low levels of an alanine-oxaloacetate aminotransferase were detected. Alanine-2-oxo-glutarate aminotransferase could not be demonstrated in this bacterium.

Utilization of nitrogen compounds and ammonia assimilation by Chromatiaceae

Chromatium vinosum strain D, Thiocapsa roseopersicina strain 6311 and Ectothiorhodospira mobilis strain 8112 were grown anaerobically in the light with various single nitrogen sources. When substituted for NH4Cl only glutamine and casamino acids supported good growth of all strains tested. Peptone and urea were utilized by C. vinosum and T. roseopersicina, glutamate, asparagine and nitrate only by C. vinosum. The strains were able to grow with molecular nitrogen; complete inhibition of this growth was observed in the presence of alanine with E. mobilis, and of alanine or asparagine with T. roseopersicina. Glutamate dehydrogenase, requiring either NADH or NADPH, NADH-linked glutamate synthase, and glutamine synthetase were demonstrated in the above organisms grown on NH4Cl.

Uptake of C4 dicarboxylates and pyruvate by Rhodopseudomonas spheroides

The uptake of C4 dicarboxylates by cells from exponential cultures of Rhodopseudomonas spheroides followed saturation kinetics at concentrations below 100 muM with Km values for succinate, malate, and fumarate of 2.7, 2.3, and 0.8, respectively. Corresponding Vmax values of 50, 52, and 67.5 nmol/min per mg of protein at 20 C were obtained. Each of these compounds interfered competitively with uptake of the others, and a common transport system appears to be involved. Fructose-grown cells took up C4 dicarboxylates only at very low rates, and pyruvate-grown cells took up C4 dicarboxylates at one-third the rates found with succinate-grown cultures. Malonate and maleate inhibited uptake less severely, and aspartate and alpha-ketoglutarate had no effect at 100-fold excess. Divalent metals stimulated uptake. Light or respiration was required for uptake, and entered materials were rapidly converted to other metabolities, notably amino acids. Pyruvate entry appeared to be mediated by several systems, of which only one could be resolved kinetically. This system had a Km of 13 muM and Vmax of 5.6 nmol/min per mg of protein at 20 C. A number of related mono- and dicarboxylates interfered with pyruvate uptake. The pyruvate uptake system was distinguishable from the C4 dicarboxylate system by the absence of divalent cation stimulation and by substrate and inhibitor specificity.