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111.
112.
Replication slippage may cause parallel evolution in the secondary structures of mitochondrial transfer RNAs 总被引:9,自引:4,他引:5
Presence of the dihydrouridine (D) stem in the mitochondrial cysteine tRNA
is unusually variable among lepidosaurian reptiles. Phylogenetic and
comparative analyses of cysteine tRNA gene sequences identify eight
parallel losses of the D-stem, resulting in D-arm replacement loops.
Sampling within the monophyletic Acrodonta provides no evidence for
reversal. Slipped-strand mispairing of noncontiguous repeated sequences
during replication or direct replication slippage can explain repeats
observed within cysteine tRNAs that contain a D-arm replacement loop. These
two mechanisms involving replication slippage can account for the loss of
the cysteine tRNA D-stem in several lepidosaurian lineages, and may
represent general mechanisms by which the secondary structures of
mitochondrial tRNAs are altered.
相似文献
113.
In a previous report, the cDNA for human proteinase inhibitor 8 (PI8) was first identified, isolated, and subcloned into a mammalian expression vector and expressed in baby hamster kidney cells. Initial studies indicated that PI8 was able to inhibit the amidolytic activity of trypsin and form an SDS-stable approximately 67-kDa complex with human thrombin [Sprecher, C. A., et al. (1995) J. Biol Chem. 270, 29854-29861]. In the present study, we have expressed recombinant PI8 in the methylotropic yeast Pichia pastoris, purified the inhibitor to homogeneity, and investigated its ability to inhibit a variety of proteinases. PI8 inhibited the amidolytic activities of porcine trypsin, human thrombin, human coagulation factor Xa, and the Bacillus subtilis dibasic endoproteinase subtilisin A through different mechanisms but failed to inhibit the Staphylococcus aureus endoproteinase Glu-C. PI8 inhibited trypsin in a purely competitive manner, with an equilibrium inhibition constant (Ki) of less than 3.8 nM. The interaction between PI8 and thrombin occurred with a second-order association rate constant (kassoc) of 1.0 x 10(5) M-1 s-1 and a Ki of 350 pM. A slow-binding kinetics approach was used to determine the kinetic constants for the interactions of PI8 with factor Xa and subtilisin A. PI8 inhibited factor Xa via a two-step mechanism with a kassoc of 7.5 x 10(4) M-1 s-1 and an overall Ki of 272 pM. PI8 was a potent inhibitor of subtilisin A via a single-step mechanism with a kassoc of 1.16 x 10(6) M-1 s-1 and an overall Ki of 8.4 pM. The interaction between PI8 and subtilisin A may be of physiological significance, since subtilisin A is an evolutionary precursor to the intracellular mammalian dibasic processing endoproteinases. 相似文献
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1. We examined the role of flooding on the leaf nutrient content of riparian trees by comparing the carbon : nitrogen : phosphorus (C : N : P) ratio of leaves and litter of Rio Grande cottonwood (Populus deltoides ssp. wislizenii) in flood and non‐flood sites along the Middle Rio Grande, NM, U.S.A. The leaf C : N : P ratio was also examined for two non‐native trees, saltcedar (Tamarix chinensis) and Russian olive (Elaeagnus angustifolia), and six species of dominant riparian arthropods. 2. Living leaves and leaf litter of cottonwoods at flood sites had a significantly lower leaf N : P ratio and higher %P compared with leaves and litter at non‐flood sites. A non‐flood site downstream from wastewater effluent had a significantly lower litter C : N ratio than all other sites, suggesting N fertilisation through ground water. The non‐native trees, saltcedar and Russian olive, had higher mean leaf N content, N : P ratio, and lower C : N ratio compared with cottonwoods across study sites. 3. Riparian arthropods ranged from 5.2 to 7.1 for C : N ratio, 56–216 for C : P ratio, and 8.9–34 for N : P ratio. C content ranged from 25 to 52% of dry mass, N content from 4.7 to 10.8%, and P content from 0.59 to 1.2%. Differences in stoichiometry between high C : nutrient leaf litter and low C : nutrient invertebrates suggests possible food‐quality constraints for detritivores. 4. These results suggest that spatial and temporal variation in the C : N : P ratio of cottonwood leaves and leaf litter is influenced by surface and subsurface hydrologic connection within the floodplain. Reach‐scale variation in the elemental composition of riparian organic matter inputs may have important implications for decomposition, nutrient cycling, and food webs in river floodplain systems. 相似文献
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GREG CRONIN JAMES H. MCCUTCHAN JR JOHN PITLICK WILLIAM M. LEWIS JR 《Freshwater Biology》2007,52(8):1587-1601
1. Discharge patterns of streams and rivers may be substantially affected by changes in water management, land use, or climate. Such hydrological alterations are likely to influence biotic processes, including overall ecosystem metabolism (photosynthesis and respiration). One regulator of aquatic ecosystem metabolism directly tied to hydrology is movement of bed sediments. 2. We propose that ecosystem metabolism can be reconstructed or predicted for any suite of hydrological conditions through the use of quantitative relationships between discharge, bed movement and metabolism. We tested this concept on a plains reach of the South Platte River in Colorado. 3. Movement of bed sediments was predicted from river discharge and the Shields stress, a ratio of velocity‐induced stress to sediment grain size. Quantitative relationships were established empirically between metabolic response to bed movement and recovery from bed movement, thus linking metabolism to hydrology. 4. The linkage of metabolism to hydrology allowed us to reconstruct daily photosynthesis and respiration over the 70‐year period for which discharge is known at our study site on the South Platte River. The reconstruction shows major ecological change caused by hydrological manipulation: the river has lost two‐thirds of its photosynthetic potential, and the ratio of photosynthesis to respiration is now much lower than it was prior to 1960. 5. The same approach could be used to anticipate ecological responses to proposed hydrological manipulations, to quantify benefits of hydrological restoration, or to illustrate potential effects of change in climate or land use on flowing‐water ecosystems. 相似文献
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SYNOPSIS. Supraspecific taxa of the ciliate Order Tintinnida Corliss, 1955 are indexed by 304 entries; genetic type species are cited with reference to the original citation. The accompanying bibliography of 133 entries includes those publications cited in the index. Authors for several type citations are corrected, as is authorship of some genera, and a new type designation is made for Parundella to replace the previous erroneous designation. Genera present in the fossil record are so indicated. 相似文献