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The filamentous bacteriophage Pf1 is structurally similar to the well known Ff (fd, fl, M13) strains, but it gives much better X-ray diffraction patterns, enabling a more detailed analysis of the molecular structure. The 46-residue protein subunit can be closely approximated by a single gently curved stretch of alpha-helix. The axes of the subunits are at a small angle to the virion axis, and several thousand subunits form an overlapping inter-digitated helical array surrounding a DNA core. We have derived a detailed model of the virion based on X-ray data and stereochemical constraints. We have considered potential sources of error in the diffraction data, and used the improved data to study regions where the protein subunit of Pf1 may deviate from a continuous alpha-helix. We use simulated annealing to escape from local minima, and various kinds of electron-density maps to guide the model building. Refinement of the model shows that the first few residues at the N terminus are non-helical, and there is a slight discontinuity in the alpha-helix near the middle of the sequence. The model is consistent both with general structural principles derived from high-resolution analysis of other proteins, and with specific chemical and spectroscopic data about Pf1. We apply the same refinement techniques to an alternative model with a non-helical surface loop between residues 13 and 19. Comparative analysis of models with and without a loop shows that the loop model is not supported by 3.3 A resolution X-ray diffraction data.

CAESP!TOL OBTUSOL A study of the cytostatic and antibacterial activity of four halogenated alicyclic monoterpenes obtained from Br sesquiterpenes isolated from two species of Laurencia; Br

Work done on construction sites is considered to be high risk, resulting in instances of occupational accidents that impact the physical, mental and social integrity of the workers as well as company productivity. Therefore, the objective of this article is to analyze the causes and consequences of occupational accidents occured in the second semester of 2012 in two construction projects in Neiva, Colombia. The approach is documentary, supported by information sources of occupational accidents reported to the Administration of Occupational Accidents (ARL in Spanish); the model developed by Frank E. Bird was used to determine the causes of these accidents. A total of 117 accidents were analyzed, detecting 195 failures of control, 136 personal factors, 112 work related factors, 151 unsafe acts and 54 unsafe conditions; some accidents had more than one cause. In this manner, the authors conclude that the largest percentage of occupational accidents were caused by lack of control, followed by unsafe acts. Therefore, the use of programs of risk management priorities is recommended, based on the proper identification of hazards, allowing the mitigation and control of risk; likewise developing a culture of self-care in workers is recommended.

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