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- ERKENS, U, et al.
(författare)
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MONITORING OF ACTIVE GALACTIC NUCLEI .6. THE QUASAR MKN-876
- 1995
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Ingår i: ASTRONOMY AND ASTROPHYSICS. - : SPRINGER VERLAG. - 0004-6361. ; 296:1, s. 90-98
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- We present the results of a 6 month monitoring campaign of Mkn 876, a low luminosity quasar with M(abs) approximate to -24. High resolution spectroscopy and direct imaging in the Johnson filters B, V, R, I was carried out in the first half of 1990. Line a
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- Erkens, U., et al.
(författare)
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Monitoring of active galactic nuclei. VI. The quasar Mrk 876
- 1995
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Ingår i: Astronomy and Astrophysics. - 0004-6361 .- 1432-0746. ; 296:1, s. 90-9898
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Tidskriftsartikel (refereegranskat)abstract
- For pt. V see ibid., vol. 285, no. 3, p. 857-67 (1994). We present the results of a 6 month monitoring campaign of Mrk 876, a low luminosity quasar with M absap-24. High resolution spectroscopy and direct imaging in the Johnson filters B, V, R, I was carried out in the first half of 1990. Line and continuum light curves are presented. Mrk 876 showed a decrease of 17% in the continuum over the whole observing period whereas the line flux of Hbeta and Halpha stayed constant. This implies, that the radius of the broad line region is larger than 6 light months. Coadded spectra of Hbeta and Halpha are presented which confirm an extreme asymmetry of the line profiles to the red. With respect to much earlier spectra the line profiles changed considerably
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- Arft, M, et al.
(författare)
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Responses of tundra plants to experimental warming : Meta-analysis of the international tundra experiment
- 1999
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Ingår i: Ecological Monographs. - 0012-9615 .- 1557-7015. ; 69:4, s. 491-511
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Tidskriftsartikel (refereegranskat)abstract
- The International Tundra Experiment (ITEX) is a collaborative, multisite experiment using a common temperature manipulation to examine variability in species response across climatic and geographic gradients of tundra ecosystems. ITEX was designed specifically to examine variability in arctic and alpine species response to increased temperature. We compiled from one to four years of experimental data from 13 different ITEX sites and used meta-analysis to analyze responses of plant phenology, growth, and reproduction to experimental warming. Results indicate that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period; however, there was little impact on growth cessation at the end of the season. Quantitative measures of vegetative growth were greatest in warmed plots in the early years of the experiment, whereas reproductive effort and success increased in later years. A shift away from vegetative growth and toward reproductive effort and success in the fourth treatment year suggests a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering. Both vegetative and reproductive responses varied among life-forms; herbaceous forms had stronger and more consistent vegetative growth responses than did woody forms. The greater responsiveness of the herbaceous forms may be attributed to their more flexible morphology and to their relatively greater proportion of stored plant reserves. Finally, warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. Greater resource investment in vegetative growth may be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and there may be little opportunity for successful germination or seedling development. In contrast, in the High Arctic, heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvegetated ground. The observed differential response to warming suggests that the primary forces driving the response vary across climatic zones, functional groups, and through time.
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- Kim, S J, et al.
(författare)
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Distinct "assisted" and "spontaneous" mechanisms for the insertion of polytopic chlorophyll-binding proteins into the thylakoid membrane
- 1999
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Ingår i: Journal of Biological Chemistry. - 0021-9258 .- 1083-351X. ; 274:8, s. 4715-4721
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Tidskriftsartikel (refereegranskat)abstract
- The biogenesis of several bacterial polytopic membrane proteins has been shown to require signal recognition particle (SRP) and protein transport machinery, and one such protein, the major light-harvesting chlorophyll-binding protein (LHCP) exhibits these requirements in chloroplasts. In this report we have used in vitro insertion assays to analyze four additional members of the chlorophyll-alb-binding protein family. We show that two members, Lhca1 and Lhcb5, display an absolute requirement for stroma, nucleoside triphosphates, and protein transport apparatus, indicating an "assisted" pathway that probably resembles that of LHCP. Two other members, however, namely an early light-inducible protein 2 (Elip2) and photosystem II subunit S (PsbS), can insert efficiently in the complete absence of SRP, SecA activity, nucleoside triphosphates, or a functional Sec system. The data suggest a possibly spontaneous insertion mechanism that, to date, has been characterized only for simple single-span proteins. Of the membrane proteins whose insertion into thylakoids has been analyzed, five have now been shown to insert by a SRP/Sec-independent mechanism, suggesting that this is a mainstream form of targeting pathway. We also show that PsbS and Elip2 molecules are capable of following either "unassisted" or assisted pathways, and we discuss the implications for the mechanism and role of SRP in chloroplasts.
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