Howard Gest

Characterization of Rhodopseudomonas capsulata

Thirty-three strains of Rhodopseudomonas capsulata have been studied in order to develop a more comprehensive characterization of the species. On the basis of morphological, nutritional, physiological and other properties, the characteristics of an “ideal biotype” have been defined, which can be used to distinguish Rps. capsulata from similar purple bacteria. In this connection, two properties of Rps. capsulata are of particular note: a) sensitivity to penicillin G is 103–105 times greater than that shown by closely related species, and b) all strains examined are susceptible to lysis by one or more strains of host species-specific virulent bacteriophages. It appears that members of the species Rps. capsulata form a stringent taxonomic grouping.

Sulfur isotope effects associated with oxidation of sulfide by O2 in aqueous solution

Normal sulfur isotope effects averaging epsilon = -5.2 +/- 1.4% (s.d.) were consistently observed for the oxidation of sulfide in aqueous solution. Reaction products were sulfate, thiosulfate and sulfite at pH 10.8-11 in distilled water; S0 was formed in two experiments with synthetic seawater at pH 8-9.5. Because the -5.2% normal isotope effect differs significantly from the previously measured +2% inverse effect associated with anaerobic oxidation of sulfide by photosynthetic bacteria, stable sulfur isotopic measurements are potentially useful for distinguishing aerobic vs. anaerobic sulfide oxidation in marine and freshwater sulfureta.

Rhodospirillum centenum, sp. nov., a thermotolerant cyst-forming anoxygenic photosynthetic bacterium

A novel non-sulfur purple photosynthetic bacterium, designated Rhodospirillum centenum, was isolated from an enrichment culture designed to favor growth of anoxygenic photosynthetic N2-fixing bacteria. R. centenum grows optimally at 40–42° C and has the capacity to produce cytoplasmic ‘R bodies’, refractile structures not observed hitherto in photosynthetic prokaryotes. The bacterium is also unusual among photosynthetic bacteria in that it forms desiccation-resistant cysts when grown aerobically in darkness with butyrate as the sole carbon source.

Gene transfer agents, bacteriophages, and bacteriocins of Rhodopseudomonas capsulata

Thirty-three wild type strains of Rhodopseudomonas capsulata were examined for ability to engage in genetic recombination through mediation by “gene transfer agent” (GTA) particles. The genetic exchange assays were based on capacity of strains to produce or receive GTA required for restoration of photosynthetic growth competence to a non-photosynthetic “white” mutant or for acquisition of resistance to rifampicin. A majority of the strains could either produce or receive GTA, and it was demonstrated that the agent is species specific. Possible relations between GTA and bacteriophages or bacteriocins were investigated. Sixteen types of virulent phages active on Rps. capsulata were isolated and their host ranges determined. Tests for transduction by the phages gave uniformly negative results. The viruses showed strict species specificity, but there was no apparent correlation between capacity of the Rps. capsulata strains to donate or receive GTA and susceptibility to the phages. A comparable survey disclosed that most of the bacterial strains were sensitive to or capable of producing bacteriocins; the latter also appear to be unrelated to GTA activity. The collection of bacterial strains was also screened for detection of lysogenic properties. None of the isolates is a “true” lysogen, but phages were detected in cultures of two strains, which may be “phage carriers” or pseudolysogens.

Generation of succinyl-coenzyme A in photosynthetic bacteria

Pathways of succinyl-Coenzyme A (succinyl-CoA) formation in various photosynthetic bacteria were investigated through several approaches, including determination of activity levels of relevant enzymes. Extracts of photosynthetically grown cells of representative Rhodospirillaceae and Chromatium vinosum showed α-ketoglutarate dehydrogenase (KGD) activities sufficient to account for generation of the succinyl-CoA required for biosynthetic metabolism. Except as noted below, the observed ratios of fumarate reductase/succinate dehydrogenase activities were low, consistent with the conclusion that these organisms produce succinyl-CoA oxidatively from α-ketoglutarate (KG), rather than by reductive metabolism of fumarate. On the other hand, the green bacterium Chlorobium limicola appears to produce succinyl-CoA by the reductive pathway; in this organism, KGD activity could not be detected, and a high fumarate reductase/succinate dehydrogenase ratio was observed. Results obtained with Rhodopseudomonas gelatinosa suggest that this otherwise typical member of the Rhodospirillaceae may be able to generate succinyl-CoA via both “arms” of the citric acid cycle, that is, oxidatively from KG, and reductively from fumarate. To further explore the several physiological roles of the conversion: KG→succinyl-CoA in Rhodopseudomonas capsulata, a mutant (strain KGD 11) almost completely blocked in KGD activity was isolated and studied in detail. Under anaerobic photosynthetic conditions, KGD 11 grows readily on succinate as the sole carbon source; in contrast to the wild type parent, however, it cannot grow with l-glutamate as the source of carbon. The R. capsulata parental strain can grow in darkness as an aerobic heterotroph on various carbon/energy sources including pyruvate, D,L-malate, or succinate. Mutant KGD 11, however, is unable to grow aerobically on the substrates noted. These results indicate that the energy for aerobic dark growth of R. capsulata is provided by ”respiratory phosphorylation” fueled by citric acid cycle function, and that this requires a substantial level of KGD activity. The present findings also indicate that citric acid cycle sequences in most of the Rhodospirillaceae prominently used in current research are geared to operate in the oxidative direction, as in nonphotosynthetic aerobic heterotrophs.

A unique photosynthetic reaction center from Heliobacterium chlorum

A previously unknown type of photosynthetic reaction center in the brownish‐green bacterium Heliobacterium chlorum is described. The reaction center is tightly bound in a highly proteinaceous undifferentiated plasma membrane and contains bacteriochlorophyll g, which has major in vivo absorbancies at 788, 576 and 370 nm. The purified membrane shows a reversible photobleaching at 798 nm; the reaction center bacteriochlorophyll g is designated P798. A reversible photobleaching at 553 nm is assigned to photooxidation of a membrane‐bound c‐type cytochrome. The membrane structure, pigment composition and photochemical properties suggest that H. chlorum may represent a fifth family of anoxygenic photosynthetic procaryotes.

Energy Conversion and Generation of Reducing Power in Bacterial Photosynthesis

Publisher Summary Bacterial and green-plant photosynthesis share a number of basic features, but are distinctly different in respect to one another. The physiological property of the photosynthetic bacteria that makes the bacteria unique in the biological world states the ability of bacteria to grow rapidly under anaerobic conditions, using light as the ultimate energy source. The photosynthetic bacteria display a remarkable metabolic diversity and versatility. Essentially, in the anabolic phase of cell growth, appropriate organic intermediates and monomers are produced from the carbon and nitrogen sources of the external medium and these are utilized for the synthesis of proteins, nucleic acids, lipids, carbohydrates, and various other complex cellular substances. Synthesis of the intermediates and monomers requires energy in the form of ATP and reducing power—primarily as reduced nicotinamide nucleotide. The chapter describes photophosphorylation, generation of net reducing power, comparison of energy metabolism and electron-transfer patterns in photosynthetic bacteria and clostridia, and the regulatory mechanisms.

Genetic transfer of nitrogenase–hydrogenase activity in Rhodopseudomonas capsulata

THE transfer of nitrogen fixation (nif) genes to bacterial mutants defective in this capacity (nif−) by transduction1 and conjugation2 was first observed in 1971 with Klebsiella pneumoniae. The investigations cited and the subsequent demonstration3 that nif genes can be transferred from K. pneumoniae to Escherichia coli have been widely regarded as harbingers of the molecular biology of the nitrogen fixation process. We now report a new system of interest in this connection, namely, the transfer of nif genes in the non-sulphur purple photosynthetic bacterium Rhodopseudomonas capsulata. Genetic transfer in this organism is mediated by a filterable vector of small size and unknown nature, the “gene transfer agent” (GTA), discovered by Marrs4.

Macroscopic phototactic behavior of the purple photosynthetic bacterium Rhodospirillum centenum

Most photosynthetic microorganisms have the capability of photosensing light quality and intensity. Movement of motile photosynthetic microorganisms to locales that offer optimal physical and chemical conditions for light-dependent growth provides obvious selective advantages. Among phototrophs, many cyanobacteria and algae migrate towards or away from the direction of light, a process termed phototaxis. In contrast, anoxygenic photosynthetic bacteria are believed to respond to changes in light intensity in a manner that is not related to the direction of light, a process that is often described by the term “photophobic”. We recently reported that “swarm colonies” of the purple photosynthetic bacterium Rhodospirillum centenum are capable of macroscopically visible phototactic behavior. In the present study we further characterize the phototactic behavior of R. centenum swarm colonies and provide an action spectrum that delineates regions of the spectrum that elicit positive and negative phototaxis.

Growth of a photosynthetic bacterium anaerobically in darkness, supported by “oxidant-dependent” sugar fermentation

Rhodopseudomonas capsulata can obtain energy for growth from light (anaerobically) and can also grow heterotrophically in darkness using alternative energy conversion modes, namely, aerobic respiration or an unusual type of anaerobic catabolism of sugars. Dark anaerobic growth with fructose as sole carbon and energy source is dependent on the presence of an “accessory” oxidant such as trimethylamine-N-oxide, is accompanied by production of lactate and other classical fermentation products, and yields cells with a high content of photosynthetic pigments and polyhydroxybutyrate.