农地流转约束与农户兼业行为

综述了农地流转的研究概况,讨论了被以往研究所忽视的约束农村土地流转的市场封闭性及土地资产专用性因素,认为农地流转市场信息不畅、充满风险、买方有限,基于专业化目的的农地流转可能为买方提供获利空间和转包激励,但因专业化而伴随农地资产在专用性程度上的增加会限制农地流转。分析了从农地流转到规模经营的过渡的历史与现实。     

Analysis of the reaction mechanism and substrate specificity of haloalkane dehalogenases by sequential and structural comparisons

Haloalkane dehalogenases catalyse environmentally importantdehalogenation reactions. These microbial enzymes representobjects of interest for protein engineering studies, attemptingto improve their catalytic efficiency or broaden their substratespecificity towards environmental pollutants. This paper presentsthe results of a comparative study of haloalkane dehalogenasesoriginating from different organisms. Protein sequences andthe models of tertiary structures of haloalkane dehalogenaseswere compared to investigate the protein fold, reaction mechanismand substrate specificity of these enzymes. Haloalkane dehalogenasescontain the structural motifs of /ß-hydrolases and epoxidaseswithin their sequences. They contain a catalytic triad withtwo different topological arrangements. The presence of a structurallyconserved oxyanion hole suggests the two-step reaction mechanismpreviously described for haloalkane dehalogenase from Xanthobacterautotrophicus GJ10. The differences in substrate specificityof haloalkane dehalogenases originating from different speciesmight be related to the size and geometry of an active siteand its entrance and the efficiency of the transition stateand halide ion stabilization by active site residues. Structurallyconserved motifs identified within the sequences can be usedfor the design of specific primers for the experimental screeningof haloalkane dehalogenases. Those amino acids which were predictedto be functionally important represent possible targets forfuture site-directed mutagenesis experiments.     

特异性抗肿瘤免疫核糖核酸的提取

用H22细胞接种于小鼠腹腔，抽取癌性腹水，分离肿瘤细胞，用肿瘤细胞裂解物加弗氏完全佐剂充分乳化制备成免疫原，接种于小鼠，取小鼠的免疫器官提取核糖核酸，并进行相关性能检测。结果表明该方法制备提取免疫的核糖核酸携带供体肿瘤特异性免疫信息，能提高抗肿瘤的疗效。     

Preference of certain scolytidae for different conifers

Nine Scolytidae (Cryphalus piceae, Cryphalus abietis, Pityokteines curvidens, Dendroctonus micans, Ips sexdentatus, Ips typographus,Orthotomicus erosus, Tomicus piniperda, andPhloeosinus bicolor) were subjected to olfaction tests on ten conifer species taken two by two. These conifers wereAbies cephalonica, Abies nordmanniana, Picea abies, Picea orientalis, Pinus pinaster, Pinus sylvestris, Pinus brutia, Pinus laricio,Cupressus atlantica, andCupressus sempervirens. A statistical study of the results, by means of the factorial analysis of correspondence completed by the duo preference test, commonly used in sensory analysis, revealed a taxonomic clustering by genus of the plant species and analogous specific attraction behavior for the insects.Pityokteines curvidens has a behavior analogous to that of the twoCryphalus considered.Phloesinus bicolor shows a very strong specificity forCupressus. The essential oils of the conifers were analyzed to determine their terpene composition and the ten odor spectra thus obtained were compared. The hierarchical classification, using a Euclidian distance, brought out similarities in the spectra, especially in the case ofPinus. It is shown that definitive establishment of Scolytidae is not due to the presence in the odor spectrum of any particular terpenoid. The attractive power of a species results from the synergism of the different terpenes. Moreover the definitive establishment of the insects also depends on their sensorial adaptation to volatile substances which can be wider or narrower for the species studied.     

A within-modality test of semantic knowledge: The Size/Weight Attribute Test.

Individuals with semantic dementia (SD; n=12) or Alzheimer's disease (AD; n=20) and healthy volunteers (n=40) were tested on tasks probing attribute rather than associative knowledge of animals and objects in the visual and verbal domains. The tasks were within modality, in that they probed knowledge in a single presentation modality (pictures or written words) and did not require cross-modal matching. Participants were required to make the same simple judgment about triads of animal stimuli ("Which is the largest and smallest?") and object stimuli ("Which is the heaviest and lightest?"). Control participants scored at ceiling on this simple task. Overall, the SD patients were significantly more impaired on this task than were the AD patients, who in turn were significantly more impaired than the controls. There was a strong trend for SD but not AD patients to show worse performance with verbal than visual material. However, no significant effects of category were found in either patient group. The Size/Weight Attribute Test has the potential to assess modality-specific deficits of semantic knowledge. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Isolation of the acidic and basic lectins from winged bean seed (Psophocarpus tetragonolobus (L.) DC)

The acidic and basic lectins of the winged bean were isolated from seed extracts by affinity chromatography on lactose-Sepharose 6B and melibiose Bio-Gel P150, respectively. The lectins isolated from six varieties of Psophocarpus tetragonolobus, collected from different regions of south-east Asia, showed no significant differences in their properties; their subunit M_r and amino acid compositions were the same as those of the acidic and basic lectins isolated by non-affinity methods. On isoelectric focusing the affinity purified acidic lectins showed two major protein components (isolectins) while the affinity purified basic lectins had four protein components. The acidic lectins showed a preference for β-D-galactosides and they agglutinated trypsinised human type A, B and O erythrocytes but not trypsinised rabbit erythrocytes. The basic lectins showed a preference for α-D-galactosides and they agglutinated trypsinised rabbit and human type A and B erythrocytes but not trypsinised human type O erythrocytes.     

Neural correlates of sexual arousal in homosexual and heterosexual men.

Men exhibit much higher levels of genital and subjective arousal to sexual stimuli containing their preferred sex than they do to stimuli containing only the nonpreferred sex. This study used event-related functional magnetic resonance imaging to investigate how this category-specific pattern would be reflected in the brains of homosexual (n = 11) and heterosexual (n = 11) men. Comparisons of activation to preferred sexual stimuli, nonpreferred sexual stimuli, and sports stimuli revealed large networks correlated with sexual arousal, spanning multiple cortical and subcortical areas. Both homosexual and heterosexual men exhibited category-specific arousal in brain activity. Within the amygdala, greater preference-related activity was observed in homosexual men, but it is unclear whether this is a cause or a consequence of their sexuality. In a subsequent analysis of regions hypothesized to support arousal, both participant groups demonstrated widespread increases in evoked activity for preferred stimuli. Aggregate data from these regions produced significant differences between stimulus types in 16 out of 22 participants. Significant activational differences matched reported sexual orientation in 15 of these 16 participants, representing an advance in psychophysiological measures of arousal. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Specific previous experience affects perception of harmony and meter.

Prior knowledge shapes our experiences, but which prior knowledge shapes which experiences? This question is addressed in the domain of music perception. Three experiments were used to determine whether listeners activate specific musical memories during music listening. Each experiment provided listeners with one of two musical contexts that was presented simultaneously with a melody. After a listener was familiarized with melodies embedded in contexts, the listener heard melodies in isolation and judged the fit of a final harmonic or metrical probe event. The probe event matched either the familiar (but absent) context or an unfamiliar context. For both harmonic (Experiments 1 and 3) and metrical (Experiment 2) information, exposure to context shifted listeners' preferences toward a probe matching the context that they had been familiarized with. This suggests that listeners rapidly form specific musical memories without explicit instruction, which are then activated during music listening. These data pose an interesting challenge for models of music perception which implicitly assume that the listener's knowledge base is predominantly schematic or abstract. (PsycINFO Database Record (c) 2011 APA, all rights reserved)     

Structure Determination of the Top‐Loop of the Conserved 3′‐Terminal Secondary Structure in the Genome of Flaviviruses

To what extent small differences in RNA sequences (mutations) can have a profound impact on biology remains an intriguing question. This effect can be studied by using untranslated RNA regions as a model. We have studied the influence of mutations on the structure of an RNA hairpin that occurs in the 3′‐untranslated region (UTR) of Flaviviridae, and is known to have a large impact on the vector dependency of flaviviruses. Three related RNA sequences were studied by NMR spectroscopy. The selected sequences represent each one of the three clusters in the flavivirus genes (mosquito‐borne, tick‐borne, and no‐known‐vector viruses). A new strategy was used to obtain chemical shift signatures of carbonyl atoms in unlabeled uridine nucleobases to characterize their involvement in hydrogen bonding. Clear differences occur in the structures and stacking pattern of the three RNA hairpins. The observed differences cannot be predicted based on sequence analysis. A different biology can be correlated with a different RNA tertiary structure. The underlying biological mechanism, however, remains to be studied.     

Decoupled Roles for the Atypical,Bifurcated Binding Pocket of the ybfF Hydrolase

Serine hydrolases have diverse intracellular substrates, biological functions, and structural plasticity, and are thus important for biocatalyst design. Amongst serine hydrolases, the recently described ybfF enzyme family are promising novel biocatalysts with an unusual bifurcated substrate‐binding cleft and the ability to recognize commercially relevant substrates. We characterized in detail the substrate selectivity of a novel ybfF enzyme from Vibrio cholerae (Vc‐ybfF) by using a 21‐member library of fluorogenic ester substrates. We assigned the roles of the two substrate‐binding clefts in controlling the substrate selectivity and folded stability of Vc‐ybfF by comprehensive substitution analysis. The overall substrate preference of Vc‐ybfF was for short polar chains, but it retained significant activity with a range of cyclic and extended esters. This broad substrate specificity combined with the substitutional analysis demonstrates that the larger binding cleft controls the substrate specificity of Vc‐ybfF. Key selectivity residues (Tyr116, Arg120, Tyr209) are also located at the larger binding pocket and control the substrate specificity profile. In the structure of ybfF the narrower binding cleft contains water molecules prepositioned for hydrolysis, but based on substitution this cleft showed only minimal contribution to catalysis. Instead, the residues surrounding the narrow binding cleft and at the entrance to the binding pocket contributed significantly to the folded stability of Vc‐ybfF. The relative contributions of each cleft of the binding pocket to the catalytic activity and folded stability of Vc‐ybfF provide a valuable map for designing future biocatalysts based on the ybfF scaffold.