Biomedical subjects

D A Bryant

Publications and source records attributed to D A Bryant.

At least 19 recordsLinked to original sources

PubMed2001

Backbone dynamics of plastocyanin in both oxidation states. Solution structure of the reduced form and comparison with the oxidized state.

A model-free analysis based on (15)N R(1), (15)N R(2), and (15)N-(1)H nuclear Overhauser effects was performed on reduced (diamagnetic) and oxidized (paramagnetic) forms of plastocyanin from Synechocystis sp. PCC6803. The protein backbone is rigid, displaying a small degree of mobility in the sub-nanosecond time scale. The loops surrounding the copper ion, involved in physiological electron transfer, feature a higher extent of flexibility in the longer time scale in both redox states, as measured from D(2)O exchange of amide protons and from NH-H(2)O saturation transfer experiments. In contrast to the situation for other electron transfer proteins, no significant difference in the dynamic properties is found between the two redox forms. A solution structure was also determined for the reduced plastocyanin and compared with the solution structure of the oxidized form in order to assess possible structural changes related to the copper ion redox state. Within the attained resolution, the structure of the reduced plastocyanin is indistinguishable from that of the oxidized form, even though small chemical shift differences are observed. The present characterization provides information on both the structural and dynamic behavior of blue copper proteins in solution that is useful to understand further the role(s) of protein dynamics in electron transfer processes.

Amino Acid Sequence↗

PubMed2001

Photosystem stoichiometry and state transitions in a mutant of the cyanobacterium Synechococcus sp. PCC 7002 lacking phycocyanin.

Phycobilisomes (PBS) function as light-harvesting antenna complexes in cyanobacteria, red algae and cyanelles. They are composed of two substructures: the core and peripheral rods. Interposon mutagenesis of the cpcBA genes of Synechococcus sp. PCC 7002 resulted in a strain (PR6008) lacking phycocyanin and thus the ability to form peripheral rods. Difference absorption spectroscopy of whole cells showed that intact PBS cores were assembled in vivo in the cpcBA mutant strain PR6008. Fluorescence induction measurements demonstrated that the PBS cores are able to deliver absorbed light energy to photosystem (PS) II, and fluorescence induction transients in the presence of DCMU showed that PR6008 cells could perform a state 2 to state 1 transition with similar kinetics to that of the wild-type cells. Thus, PBS core assembly, light-harvesting functions and energy transfer to PS I were not dependent upon the assembly of the peripheral rods. The ratio of PS II:PS I in the PR6008 cells was significantly increased, nearly twice that of the wild-type cells, possibly a result of long-term adaptation to compensate for the reduced antenna size of PS II. However, the ratio of PBS cores:chlorophyll remained unchanged. This result indicates that approximately half of the PS II reaction centers in the PR6008 cells had no closely associated PBS cores.

Bacterial Proteins↗

PubMed2001

Electron transfer may occur in the chlorosome envelope: the CsmI and CsmJ proteins of chlorosomes are 2Fe-2S ferredoxins.

Chlorosomes of the green sulfur bacterium Chlorobium tepidum have previously been shown to contain at least 10 polypeptides [Chung, S., Frank, G., Zuber, H., and Bryant, D. A. (1994) Photosynth. Res. 41, 261-275]. Based upon the N-terminal amino acid sequences determined for two of these proteins, the corresponding genes were isolated using degenerate oligonucleotide hybridization probes. The csmI and csmJ genes encode proteins of 244 and 225 amino acids, respectively. A third gene, denoted csmX, that predicts a protein of 221 amino acids with strong sequence similarity to CsmI and CsmJ, was found to be encoded immediately upstream from the csmJ gene. All three proteins have strong sequence similarity in their amino-terminal domains to [2Fe-2S] ferredoxins of the adrenodoxin/putidaredoxin subfamily of ferredoxins. CsmI and CsmJ were overproduced in Escherichia coli, and both proteins were shown by EPR spectroscopy to contain iron-sulfur clusters. The g-tensor and relaxation properties are consistent with their assignment as [2Fe-2S] clusters. Isolated chlorosomes were also shown to contain [2Fe-2S] clusters whose properties were similar to those of the recombinant CsmI and CsmJ proteins. Redox titration of isolated chlorosomes showed these clusters to have potentials of about -201 and +92 mV vs SHE. The former potential is similar to that measured by redox titration of the clusters in inclusion bodies of CsmJ. Possible roles for these iron-sulfur proteins in electron transport and light harvesting are discussed.

Amino Acid Sequence↗

PubMed2001

Chromosomal gene inactivation in the green sulfur bacterium Chlorobium tepidum by natural transformation.

Conditions for inactivating chromosomal genes of Chlorobium tepidum by natural transformation and homologous recombination were established. As a model, mutants unable to perform nitrogen fixation were constructed by interrupting nifD with various antibiotic resistance markers. Growth of wild-type C. tepidum at 40 degrees C on agar plates could be completely inhibited by 100 microg of gentamicin ml(-1), 2 microg of erythromycin ml(-1), 30 microg of chloramphenicol ml(-1), or 1 microg of tetracycline ml(-1) or a combination of 300 microg of streptomycin ml(-1) and 150 microg of spectinomycin ml(-1). Transformation was performed by spotting cells and DNA on an agar plate for 10 to 20 h. Transformation frequencies on the order of 10(-7) were observed with gentamicin and erythromycin markers, and transformation frequencies on the order of 10(-3) were observed with a streptomycin-spectinomycin marker. The frequency of spontaneous mutants resistant to gentamicin, erythromycin, or spectinomycin-streptomycin was undetectable or significantly lower than the transformation frequency. Transformation with the gentamicin marker was observed when the transforming DNA contained 1 or 3 kb of total homologous flanking sequence but not when the transforming DNA contained only 0.3 kb of homologous sequence. Linearized plasmids transformed at least an order of magnitude better than circular plasmids. This work forms a foundation for the systematic targeted inactivation of genes in C. tepidum, whose 2.15-Mb genome has recently been completely sequenced.

Chlorobi↗

PubMed2000

Recruitment of a foreign quinone into the A(1) site of photosystem I. I. Genetic and physiological characterization of phylloquinone biosynthetic pathway mutants in Synechocystis sp. pcc 6803.

Genes encoding enzymes of the biosynthetic pathway leading to phylloquinone, the secondary electron acceptor of photosystem (PS) I, were identified in Synechocystis sp. PCC 6803 by comparison with genes encoding enzymes of the menaquinone biosynthetic pathway in Escherichia coli. Targeted inactivation of the menA and menB genes, which code for phytyl transferase and 1,4-dihydroxy-2-naphthoate synthase, respectively, prevented the synthesis of phylloquinone, thereby confirming the participation of these two gene products in the biosynthetic pathway. The menA and menB mutants grow photoautotrophically under low light conditions (20 microE m(-2) s(-1)), with doubling times twice that of the wild type, but they are unable to grow under high light conditions (120 microE m(-2) s(-1)). The menA and menB mutants grow photoheterotrophically on media supplemented with glucose under low light conditions, with doubling times similar to that of the wild type, but they are unable to grow under high light conditions unless atrazine is present to inhibit PS II activity. The level of active PS II per cell in the menA and menB mutant strains is identical to that of the wild type, but the level of active PS I is about 50-60% that of the wild type as assayed by low temperature fluorescence, P700 photoactivity, and electron transfer rates. PS I complexes isolated from the menA and menB mutant strains contain the full complement of polypeptides, show photoreduction of F(A) and F(B) at 15 K, and support 82-84% of the wild type rate of electron transfer from cytochrome c(6) to flavodoxin. HPLC analyses show high levels of plastoquinone-9 in PS I complexes from the menA and menB mutants but not from the wild type. We propose that in the absence of phylloquinone, PS I recruits plastoquinone-9 into the A(1) site, where it functions as an efficient cofactor in electron transfer from A(0) to the iron-sulfur clusters.

Alkyl and Aryl Transferases↗

PubMed2000

Paramagnetic 1H NMR spectroscopy of the reduced, unbound photosystem I subunit PsaC: sequence-specific assignment of contact-shifted resonances and identification of mixed- and equal-valence Fe-Fe pairs in [4Fe-4S] centers FA- and FB-.

The PsaC subunit of Photosystem I (PS I) is a 9.3-kDa protein that binds two important cofactors in photosynthetic electron transfer: the [4Fe-4S] clusters FA and FB. The g-tensor orientation of FA- and FB- is believed to be correlated to the preferential localization of the mixed-valence and equal-valence (ferrous) iron pairs in each [4Fe-4S]+ cluster. The preferential position of the mixed-valence and equal-valence pairs, in turn. can be inferred from the study of the temperature dependence of contact-shifted resonances by 1H NMR spectroscopy. For this, a sequence-specific assignment of these signals is required. The 1H NMR spectrum of reduced, unbound PsaC from Synechococcus sp. PCC 7002 at 280.4 K in 99% D2O solution shows 18 hyperfine-shifted resonances. The non-solvent-exchangeable, hyperfine-shifted resonances of reduced PsaC are clearly identified as belonging to the cysteines coordinating the clusters FA- and FB- by their downfield chemical shifts, by their temperature dependencies, and by their short T1 relaxation times. The usual fast method of assigning the 1H NMR spectra of reduced [4Fe-4S] proteins through magnetization transfer from the oxidized to the reduced state was not feasible in the case of reduced PsaC. Therefore, a de novo self-consistent sequence-specific assignment of the hyperfine-shifted resonances was obtained based on dipolar connectivities from 1D NOE difference spectra and on longitudinal relaxation times using the X-ray structure of Clostridium acidi urici 2[4Fe-4S] cluster ferredoxin at 0.94 A resolution as a model. The results clearly show the same sequence-specific distribution of Curie and anti-Curie cysteines for unbound, reduced PsaC as established for other [4Fe-4S]-containing proteins; therefore, the mixed-valence and equal-valence (ferrous) Fe-Fe pairs in FA- and FB- have the same preferential positions relative to the protein. The analysis reveals that the magnetic properties of the two [4Fe-4S] clusters are essentially indistinguishable in unbound PsaC, in contrast to the PsaC that is bound as a component of the PS I complex.

Cyanobacteria↗

PubMed1999

The solution structure of photosystem I accessory protein E from the cyanobacterium Nostoc sp. strain PCC 8009.

PsaE is a small basic subunit located on the stromal (cytoplasmic) side of photosystem I. In cyanobacteria, this subunit participates in cyclic electron transport and modulates the interactions of the complex with soluble ferredoxin. The PsaE protein isolated from the cyanobacterium Synechococcus sp. strain PCC 7002 adopts the beta topology of an SH3 domain, with five beta strands (betaA through betaE) and a turn of 3(10) helix between strands betaD and betaE [Falzone, C. J., Kao, Y.-H., Zhao, J., Bryant, D. A., and Lecomte, J. T. J. (1994) Biochemistry 33, 6052-6062]. The primary structure of the PsaE protein is strongly conserved across all oxygen-evolving photosynthetic organisms. However, variability in loop lengths, as well as N- or C-terminal extensions, suggests that the structure of a second representative PsaE subunit would be useful to characterize the interactions among photosystem I polypeptides. In this work, the solution structure of PsaE from the cyanobacterium Nostoc sp. strain PCC 8009 was determined by NMR methods. Compared to PsaE from Synechococcus sp. strain PCC 7002, this PsaE has a seven-residue deletion in the loop connecting strands betaC and betaD, and an eight-residue C-terminal extension. Angular and distance restraints derived from homonuclear and heteronuclear NMR experiments were used to calculate structures by a distance-geometry/simulated-annealing protocol. A family of 20 structures (rmsd of 0.24 A in the regular secondary structure) is presented. Differences between the two cyanobacterial proteins are mostly confined to the CD loop region; the C-terminal extension is disordered. The thermodynamic stability of Nostoc sp. strain PCC 8009 PsaE toward urea denaturation was measured by circular dichroism and fluorescence spectroscopy, and thermal denaturation was monitored by UV absorption spectroscopy. Chemical and thermal denaturation curves are modeled satisfactorily with two-state processes. The DeltaG degrees of unfolding at room temperature is 12.4 +/- 0.3 kJ mol(-1) (pH 5), and the thermal transition midpoint is 59 +/- 1 degrees C (pH 7). Interactions with other proteins in the photosystem I complex may aid in maintaining PsaE in its native state under physiological conditions.

Amino Acid Sequence↗

PubMed1999

Nitrate transport and not photoinhibition limits growth of the freshwater Cyanobacterium synechococcus species PCC 6301 at low temperature.

The effect of low temperature on cell growth, photosynthesis, photoinhibition, and nitrate assimilation was examined in the cyanobacterium Synechococcus sp. PCC 6301 to determine the factor that limits growth. Synechococcus sp. PCC 6301 grew exponentially between 20 degreesC and 38 degreesC, the growth rate decreased with decreasing temperature, and growth ceased at 15 degreesC. The rate of photosynthetic oxygen evolution decreased more slowly with temperature than the growth rate, and more than 20% of the activity at 38 degreesC remained at 15 degreesC. Oxygen evolution was rapidly inactivated at high light intensity (3 mE m-2 s-1) at 15 degreesC. Little or no loss of oxygen evolution was observed under the normal light intensity (250 microE m-2 s-1) for growth at 15 degreesC. The decrease in the rate of nitrate consumption by cells as a function of temperature was similar to the decrease in the growth rate. Cells could not actively take up nitrate or nitrite at 15 degreesC, although nitrate reductase and nitrite reductase were still active. These data demonstrate that growth at low temperature is not limited by a decrease in the rate of photosynthetic electron transport or by photoinhibition, but that inactivation of the nitrate/nitrite transporter limits growth at low temperature.

Anion Transport Proteins↗

PubMed1999

A novel nitrate/nitrite permease in the marine Cyanobacterium synechococcus sp. strain PCC 7002.

The nrtP and narB genes, encoding nitrate/nitrite permease and nitrate reductase, respectively, were isolated from the marine cyanobacterium Synechococcus sp. strain PCC 7002 and characterized. NrtP is a member of the major facilitator superfamily and is unrelated to the ATP-binding cassette-type nitrate transporters that previously have been described for freshwater strains of cyanobacteria. However, NrtP is similar to the NRT2-type nitrate transporters found in diverse organisms. An nrtP mutant strain consumes nitrate at a 4.5-fold-lower rate than the wild type, and this mutant grew exponentially on a medium containing 12 mM nitrate at a rate approximately 2-fold lower than that of the wild type. The nrtP mutant cells could not consume nitrite as rapidly as the wild type at pH 10, suggesting that NrtP also functions in nitrite uptake. A narB mutant was unable to grow on a medium containing nitrate as a nitrogen source, although this mutant could grow on media containing urea or nitrite with rates similar to those of the wild type. Exogenously added nitrite enhanced the in vivo activity of nitrite reductase in the narB mutant; this suggests that nitrite acts as a positive effector of nitrite reductase. Transcripts of the nrtP and narB genes were detected in cells grown on nitrate but were not detected in cells grown on urea or ammonia. Transcription of the nrtP and narB genes is probably controlled by the NtcA transcription factor for global nitrogen control. The discovery of a nitrate/nitrite permease in Synechococcus sp. strain PCC 7002 suggests that significant differences in nutrient transporters may occur in marine and freshwater cyanobacteria.

Amino Acid Sequence↗

PubMed1998

Measurement of photosystem I activity with photoreduction of recombinant flavodoxin.

Flavodoxin can function as an alternative electron acceptor for photosystem I (PSI) in place of ferredoxin under iron-limiting conditions. The isiB gene, encoding the flavodoxin in Synechococcus sp. PCC 7002, was overexpressed in Escherichia coli. Under the conditions employed, most recombinant flavodoxin (rFlvd) was in soluble form with cofactor correctly inserted. The absorption spectrum of rFlvd was identical to that of the native flavodoxin of the cyanobacteria. Photoreduction of rFlvd by PSI particles and thylakoid membranes was determined directly by monitoring the absorption change at 467 nm. The optimal conditions for rFlvd photoreduction were determined. Compared to other methods currently employed to measure PSI activity such as oxygen uptake in the presence of methyl viologen and NADP+ photoreduction in the presence of ferredoxin and ferredoxin:NADP+ oxidoreductase, measurement of PSI activity with flavodoxin as an electron acceptor has several advantages. It measures the full-chain electron transfer chain of PSI since flavodoxin accepts electrons from FA/FB and it is much simpler than the method with NADP+ photoreduction. With this method, we found that the affinity of wild-type PSI for rFlvd was 35% higher than that of the PsaE-less PSI, showing that this method is sensitive to structural changes of PSI. Our results demonstrate that rFlvd photoreduction is an effective and simple method for PSI activity measurement.

Bacterial Proteins↗

PubMed1998

Insertional inactivation studies of the csmA and csmC genes of the green sulfur bacterium Chlorobium vibrioforme 8327: the chlorosome protein CsmA is required for viability but CsmC is dispensable.

Targeted mutagenesis was used to investigate the roles of the CsmA and CsmC proteins of the chlorosomes of the green bacteria Chlorobium tepidum and Chlorobium vibrioforme 8327. Under the photoautotrophic growth conditions employed, CsmA is required for the viability of the cells but CsmC is dispensable. The absence of CsmC caused a small red shift in the near-infrared absorption maximum of bacteriochlorophyll d in whole cells and chlorosomes, but chlorosomes were assembled in and could be isolated from the csmC mutant. The doubling time of the csmC mutant was approximately twice that of the wild-type strain. Fluorescence emission measurements suggested that energy transfer from the bulk bacteriochlorophyll d to another pigment, perhaps bacteriochlorophyll a, emitting at 800-804 nm, was less efficient in the csmC mutant cells than in wild-type cells. These studies establish that transformation and homologous recombination can be employed in targeted mutagenesis of Chlorobium sp. and further demonstrate that chlorosome proteins play important roles in the structure and function of these light-harvesting organelles.

Bacterial Proteins↗

PubMed1998

Structure and properties in solution of PsaD, an extrinsic polypeptide of photosystem I.

PsaD is a small, extrinsic polypeptide located on the stromal side (cytoplasmic side in cyanobacteria) of the photosystem I reaction centre complex. The gene from the cyanobacterium Nostoc sp. PCC 8009 was expressed in Escherichia coli and the structure of the recovered protein in solution investigated. Size-exclusion chromatography, dynamic light scattering and measurement of 15N transverse relaxation times showed that the protein is a stable dimer in solution, whereas in the reaction centre complex it is a monomer. NMR experiments showed that the dimer is symmetrical and that there are at least two domains, one structured and the remainder unstructured. The structured domain contains a small amount of beta-sheet. Three-dimensional heteronuclear NMR spectra of [13C, 15N]PsaD showed that the structured domain is associated with the central part of the sequence while the N- and C-terminal regions are mobile. Evidence was obtained for a shift in equilibrium between two slightly different conformational states at about pH 6, and the protein was shown to bind to PsaE preferentially at neutral pH. Addition of trifluoroethanol was shown to induce the formation of a small amount of alpha-helix, and the form present in 30% trifluoroethanol appears to be more closely related to the in situ structure, which has been reported to contain one short helix in crystals [Schubert, W.-D., Klukas, O., Krauss, N., Saenger, W., Fromme, P. & Witt, H. T. (1997) J. Mol. Biol. 272, 741-769]. The significance of these findings for the assembly of the complex is discussed.

Amino Acid Sequence↗

PubMed1998

The phylogenetic relationships of Chlorobium tepidum and Chloroflexus aurantiacus based upon their RecA sequences.

Using RecA as the phylogenetic marker, the relationships of the green sulfur bacterium Chlorobium tepidum and the green non-sulfur bacterium Chloroflexus aurantiacus to other eubacteria were investigated. The recA genes of the two organisms were cloned, and the resulting protein sequences aligned with 86 other eubacterial RecA sequences. Cb. tepidum was placed as the nearest relative to the Cytophaga/Flexibacter/Bacteriodes group, a relationship supported by results obtained with several phylogenetic markers. Cf. aurantiacus was placed near Chlamydia trachomatis and the high-GC Gram-positives; however, this branching pattern was not strongly supported statistically by bootstrap analyses. Possible reasons for this ambiguity are discussed.

Amino Acid Sequence↗

PubMed1998

Growth at low temperature causes nitrogen limitation in the cyanobacterium Synechococcus sp. PCC 7002.

The coloration of cells of the cyanobacterium Synechococcus sp. PCC 7002 changed from normal blue-green to yellow-green when cells were grown at 15 degrees C in a medium containing nitrate as the sole nitrogen source. This change of coloration was similar to a general response to nutrient deprivation (chlorosis). For the chlorotic cells at 15 degrees C, the total amounts of phycobiliproteins and chlorophyll a decreased, high levels of glycogen accumulated, and growth was arithmetic rather than exponential. These changes in composition and growth occurred in cells grown at low (50 microE m-2 s-1) as well as high (250 microE m-2 s-1) light intensity. After a temperature shift-up to 38 degrees C, chlorotic cells rapidly regained their normal blue-green coloration and normal exponential growth rate within 7 h. When cells were grown at 15 degrees C in a medium containing urea as the reduced nitrogen source, cells grew exponentially and the symptoms of chlorosis were not observed. The decrease in photosynthetic oxygen evolution activity at low temperature was much smaller than the decrease in growth rate for cells grown on nitrate as the nitrogen source. These studies demonstrate that low-temperature-induced chlorosis of Synechococcus sp. PCC 7002 is caused by nitrogen limitation and is not the result of limited photosynthetic activity or photodamage to the photosynthetic apparatus, and that nitrogen assimilation is an important aspect of the low-temperature physiology of cyanobacteria.

Bacterial Proteins↗

PubMed1998

Alteration of low-temperature susceptibility of the cyanobacterium Synechococcus sp. PCC 7002 by genetic manipulation of membrane lipid unsaturation.

Cyanobacteria acclimate to low temperature by desaturating their membrane lipids. Mutant strains of Synechococcus sp. PCC 7002 containing insertionally inactivated desA (Delta12 acyl-lipid desaturase) and desB (omega3 acyl-lipid desaturase) genes were produced, and their low-temperature susceptibility was characterized. The desA mutant synthesized no linoleic acid or alpha-linolenic acid, and the desB mutant did not produce alpha-linolenic acid. The desA mutant grew more slowly than the wild-type at 22 degrees C and could not grow at 15 degrees C. The desB mutant could not continuously grow at 15 degrees C, although no observable phenotype appeared at higher temperatures. It has been shown that expression of the desA gene occurs at 38 degrees C and is up-regulated at 22 degrees C, and that the desB gene is only expressed at 22 degrees C. These results indicate that the expression of the desA and desB genes occurs at higher temperatures than those at which a significant decline in physiological activities is caused by the absence of their products. The temperature dependency of photosynthesis was not affected by these mutations. Since chlorosis and inability to grow at 15 degrees C with nitrate was suppressed by the substitution of urea as a nitrogen source, it is very likely that the chilling susceptibility of the desaturase mutants is attributable to nutrient limitation.

Adaptation, Physiological↗

PubMed1998

Characterization of the alternative sigma-factors SigD and SigE in Synechococcus sp. strain PCC 7002. SigE is implicated in transcription of post-exponential-phase-specific genes.

The sigD and sigE genes, which encode two alternative sigma-factors from the unicellular marine cyanobacterium Synechococcus sp. PCC 7002, were cloned and characterized. Strains in which the sigD and sigE genes were insertionally inactivated were viable under standard laboratory conditions, indicating that SigD and SigE are group 2 sigma-factors. When stationary-phase cells were diluted into fresh growth medium, it was observed that the sigE mutant strain required longer times to re-establish exponential growth than the wild-type strain. By monitoring the growth rates in such dilution experiments, it was observed that the lag times for the mutant strain became progressively longer as the original cultures progressed towards stationary phase. Transcripts for the sigE gene initially increased and subsequently decreased as cells grew further into stationary phase. It was determined that a functional SigE protein is required for the expression of the starvation-induced protein DpsA/PexB. The results suggest that SigE is involved in the transcription of genes specifically expressed in the post-exponential phase.

Amino Acid Sequence↗

PubMed1998

Characterization of the group 1 and group 2 sigma factors of the green sulfur bacterium Chlorobium tepidum and the green non-sulfur bacterium Chloroflexus aurantiacus.

The group 1 and group 2 sigma70-type sigma factors of the green sulfur bacterium Chlorobium tepidum and of the green nonsulfur bacterium Chloroflexus aurantiacus were cloned and characterized. Cb. tepidum was found to contain one sigma70-type sigma factor; the expression of the gene was analyzed by Northern blot hybridization and primer-extension mapping. Cf. aurantiacus has genes encoding four sigma factors of groups 1 and 2. The expression of these genes was examined in cells grown aerobically and anaerobically. The sigC gene was expressed at approximately equal levels under both conditions, resulting in its designation as the group 1 sigma factor of this organism. The only other detectable transcripts arose from the sigB gene, which was expressed at higher levels during aerobic growth. A phylogenetic tree was obtained using the group 1 sigma factors of Cb. tepidum, Cf. aurantiacus, and diverse eubacteria as the molecular marker. The resulting phylogenetic tree shows that Cb. tepidum and Cf. aurantiacus are related to each other and to the cyanobacteria. The relationship of the group 2 sigma factors of Cf. aurantiacus and the cyanobacteria was more specifically examined phylogenetically. The group 2 sigma factors of Cf. aurantiacus probably arose by gene duplication events after the split of the green nonsulfur bacteria from other photosynthetic eubacteria.

Bacterial Proteins↗

PubMed1998

Growth on urea can trigger death and peroxidation of the cyanobacterium Synechococcus sp. strain PCC 7002.

Laboratory conditions have been identified that cause the rapid death of cultures of cyanobacteria producing urease. Once the death phase had initiated in the stationary growth phase, cells were rapidly bleached of all pigmentation. Null mutations in the ureC gene, encoding the alpha subunit of urease, were constructed, and these mutants were no longer sensitive to growth in the presence of urea. High levels of peroxides, including lipid peroxides, were detected in the bleaching cells. Exogenously added polyunsaturated fatty acids triggered a similar death response. Vitamin E suppressed the formation of peroxides and delayed the onset of cell bleaching. The results suggest that these cyanobacterial cells undergo a metabolic imbalance that ultimately leads to oxidative stress and lipid peroxide formation. These observations may provide insights into the mechanism of sudden cyanobacterial bloom disappearance in nature.

Cyanobacteria↗