Amino-terminal methionine processing of the protein HPr in Streptococcus salivarius grown in continuous culture

Abstract HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Streptococci possess two forms of HPr which differ by the presence or the absence of the N-terminal methionine (Met). These forms are called HPr-1 (without Met) and HPr-2 (with Met). In order to determine whether the ratio of these two forms varies with growth conditions, we measured the amount of HPr-1 and HPr-2 present in Streptococcus salivarius grown in continuous culture at pH 7.5. The results indicated that the HPr-1/HPr-2 ratio: 1) was not related to the cellular amount of total HPr; 2) was highest (10.2±3.5) under glucose (a PTS sugar) limitation (10 mM) and low dilution rate (D = 0.1 h⁻¹; g = 6.9 h); 3) was decreased 2.4- to 5.7-fold when the amount of glucose and/or D was increased; 4) was not influenced by D when cells were cultured on galactose (a non-PTS sugar) but was two-fold higher under conditions of galactose excess (200 mM). We suggest that the cleavage of the N-terminal HPr Met is not a stochastic phenomenon but is dictated by growth conditions.     

Photodichroic studies of the photoreaction center from Rhodospirillum Rubrum. I. Attribution of P870 to two non parallel dipoles

A randomly oriented sample of photoreaction center prepared from Rhodospirillum rubrum was excited at 77 °K by an actinic linearly polarized light of 870 nm. Under such conditions, only those chromophores with components of their absorption dipoles oriented parallel to the polarization of the actinic light are bleached. The change in absorbance at 900 nm of this photoselected sample was observed while varying the angle of polarization of a weak measuring light. The polarization of the absorbance change was thus evaluated as 0.25.

This value is interpreted to mean that P₈₇₀ is attributable to two absorption dipoles forming an angle included between 35.75° and 90°. Comparison with the p value of 0.5 obtained on a similar preparation by polarization of fluorescence (Ebrey, T. G. and Clayton, R. K. (1969) Photochem. Photobiol. 10, 109–117) leads to the conclusion that either these two dipoles emit fluorescence without being coupled by singlet-singlet energy transfer or that only one of them is a fluorescence emitter in the absence of reversible singlet-singlet energy transfer.     

Photosynthetic unit size and electron-transport chain in a photoreaction center-depleted mutant of Rhodospirillum rubrum

The aim of this work was to explain the relatively fast growth of a mutant of Rhodospirillum rubrum (F24.1) which contains 7–8% of an apparently normal photoreaction center. We explored the double hypothesis that the size of its photosynthetic unit is larger than that of the wild type and that its electron-transport chain is organized in a network rather than in isolated loops. The first feature would allow faster growth under less than saturating light intensities and the second would allow faster maximal electron fluxes than would be predicted from the photoreaction center content. With respect to the first possibility, measurements of absorbance changes at 793 nm induced by short flashes of increasing intensity indicate that the photosynthetic unit of strain F24.1 is 5.6-fold larger than that of strain S1. The second possibility was verified by measuring relative electron fluxes at the photoreaction center in the two strains. This was established in the steady state from the amount of primary donor oxidized by a continuous light beam of increasing intensity. This electron flux was found to be about 70% as high in strain F24.1 as in strain S1. A more detailed study of the electron-transport chain indicated that cytochrome c₂ is by far the main secondary electron donor in strain F24.1. No evidence could be obtained for the existence of another secondary donor in that strain. The mole ratio of cytochrome c₂ to photoreaction center is about 6 in strain F24.1 as conpared to about 0.5 in strain S1. In strain 24.1, the pool of secondary donor appears to be collectively involved in the reduction of the oxidized primary donor. The replacement time at the photoreaction center of a first equivalent of oxidized cytochrome c₂ by a second equivalent of reduced cytochrome c₂ is less than or equal to 0.2 ms. The effect of the photoreaction center content on the size of the photosynthetic unit is discussed in terms of the different models proposed for the organisation of the photosynthetic unit. We propose that the electron-transport chain is organized in a network, perhaps by virtue of the lateral mobility of some of the electron carriers such as ubiquinone and cytochrome c₂.     

Circular dichroism spectra of oriented photoreaction center from Rhodospirillum rubrum

The circular dichroism spectra of oriented and unoriented photoreaction centers of Rhodospirillum rubrum are compared. Orientation is achieved by pressing photoreaction center suspended in polyacrylamide gel. The biphasic bands at 870 and 810 nm and at 630 and 600 nm undergo a rotatory strength decrease when measured in the direction of the pressure, but not when measured in the direction normal to the pressure. Such a decrease in oriented photoreaction center is consistent with the model according to which these bands are dimer exciton bands of the special pair bacteriochlorophyll.     

The Effects of Aging on Researchers' Publication and Citation Patterns

The average age at which U.S. researchers receive their first grant from NIH has increased from 34.3 in 1970, to 41.7 in 2004. These data raise the crucial question of the effects of aging on the scientific productivity and impact of researchers. Drawing on a sizeable sample of 6,388 university professors in Quebec who have published at least one paper between 2000 and 2007, our results identify two turning points in the professors'' careers. A first turning point is visible at age 40 years, where researchers start to rely on older literature and where their productivity increases at a slower pace—after having increased sharply since the beginning of their career. A second turning point can be seen around age 50, when researchers are the most productive whereas their average scientific impact is at its lowest. Our results also show that older professors publish fewer first-authored papers and move closer to the end of the list of co-authors. Although average scientific impact per paper decreases linearly until about age 50, the average number of papers in highly cited journals and among highly cited papers rises continuously until retirement. Our results show clearly that productivity and impact are not a simple and declining function of age and that we must take into account the collaborative aspects of scientific research. Science is a collective endeavor and, as our data shows, researchers of all ages play a significant role in its dynamic.     

Structure and function of carotenoids in the photoreaction center from Rhodospirillum rubrum

The bacteriochlorophyll (P-800 and P-870) of the carotenoidless photoreaction center isolated from Rhodospirillum rubrum (strain G9) is bleached irreversibly when the preparations are exposed to intense near infrared light in the presence of oxygen. This effect is much smaller in preparations, extracted from the wild type, which contain, as shown earlier, 1 mol of spirilloxanthin per mol of P-870. This photodynamic effec is shown to be due to singlet O₂. The oxidation of adrenaline in the presence of superoxide dismutase and the oxidation of 1,3-diphenylisobenzofuran are used as reporter reactions. Singlet oxygen is presumably generated by the triplet-triplet energy transfer ³bacteriochlorophyll → O₂ (³Σ).Four purified bacterial carotenoids, spirilloxanthin, sphaeroidene, sphaeroidenone and chloroxanthin were attached onto the carotenoidless photoreaction center from strain G9 in nearly 1 : 1 mol ratios with respect to P-870. Once fixed, these carotenoids confer protection against the photodynamic bleaching of bacteriochlorophyll. The relative photoprotection efficiency was 1.0 for spirilloxanthin and sphaeroidene, 0.4 for chloroxanthin and 0.2 for sphaeroidenone. The fixed carotenoids display optical activity and their molar ellipticity appears to be correlated with their relative photoprotection efficiency. The efficiency of energy transfer to P-870 is 0.90 for sphaeroidene, 0.35 for sphaeroidenone, 0.30 for chloroxanthin and 0.20 for spirilloxanthin. The energy transfer efficiency from the carotenoids to bacteriochlorophyll is suggested to be governed by the rate of the internal conversion processes of the excited singlet state of the carotenoids.A study of the difference absorption and CD spectra of the reconstituted minus carotenoidless preparations leads to the interpretation that the fixed carotenoids are in a central monocis conformation.     

Oxido-reduction of B800-850 and B880 holochromes isolated from three species of photosynthetic bacteria as studied by electron-paramagnetic resonance and optical spectroscopy

Certain redox properties of bacteriochlorophyll alpha were used to probe the structure of several light-harvesting pigment-protein complexes or holochromes. To attribute redox properties unequivocally to a given holochrome, we worked with purified holochromes. We developed purification procedures for the B880 holochromes from Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Ectothiorhodospira sp. and for the B800-850 holochromes from the latter two species. In all these holochromes, bacteriochlorophyll alpha could be oxidized by ferricyanide as witnessed by the bleaching of their near-infrared absorption bands. However, only in B880 holochromes was this oxidation reversible. Another important difference between the B800-850 and the B880 holochromes is that oxidation of the latter gives rise to a g = 2.0025 electron paramagnetic resonance (EPR) signal with linewidth varying, according to species, from 0.37 mT to 0.48 mT. Both the reversible EPR signal and absorption changes titrate with a midpoint redox potential (pH 8.0) of approximately 570 mV. Linewidth narrowing can be interpreted by delocalization of the free electron spin over approximately 12 bacteriochlorophyll molecules. While the B880 holochromes from the three species considered had indistinguishable redox properties, the B800-850 holochromes differed from one another by their circular dichroic spectra and by the relative ease of oxidation of their 800-nm and 850-nm bands. This indicates that, contrary to the B880 holochromes, the B800-850 holochromes may not form a homogeneous class.     

Photoreaction center of photosynthetic bacteria. 1. Further chemical characterization of the photoreaction center from Rhodospirillum rubrum.

The photoreaction center from Rhodospirillum rubrum contains about 90% protein, 6% pigment, mere traces of lipids, and no cytochromes. It also contains at least 1 mol of ubiquinone and 1 iron atom per mol. Its three-component polypeptide chains were isolated by preparative electrophoresis, and their molar stoichiometry was established as 1:1:1. The amino acid composition of the photoreaction center from strain S1 and from its subunits is reported. The protein as a whole contains about 65% nonpolar residues, and the degree of hydrophobicity of its subunits is alpha less than beta less than gamma. The minimal molecular weight based on the extinction coefficient and on the amino acid content is 90 000. This corresponds to a half-cystine mole number of 6.     

Association between median episiotomy and severe perineal lacerations in primiparous women

OBJECTIVE: To evaluate the association between median episiotomy and severe (third- and fourth-degree) perineal lacerations in primiparous women. DESIGN: Retrospective cohort study. SETTING: University-affiliated hospital providing secondary obstetric care in Quebec City. PATIENTS: A total of 6522 primiparous women who gave birth vaginally to a single live baby in cephalic position between 1985 and 1993. OUTCOME MEASURE: Incidence of third- and fourth-degree perineal lacerations. RESULTS: Median episiotomy was performed in 4390 women (67.3%). A total of 1002 women (15.4%) had a third- or fourth-degree laceration. The frequency of severe perineal lacerations was 20.6% with episiotomy and 4.5% without episiotomy (relative risk [RR] 4.58, 95% confidence interval [CI] 3.74-5.62). This association persisted after adjustment by stratified analysis for type of delivery and birth weight (RR 3.03, 95% CI 2.52-3.63) and by logistic regression for type of delivery, birth weight, epidural analgesia, shoulder dystocia, baby''s head circumference, experience of the physician and year of delivery (odds ratio 3.58, 95% CI 2.84-4.50). CONCLUSION: Median episiotomy is strongly associated with third- and fourth-degree perineal lacerations in primiparous women. Reducing the use of this procedure could decrease the occurrence of severe perineal tears.