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Attachment of the RNA degradosome to the bacterial inner cytoplasmic membrane prevents wasteful degradation of rRNA in ribosome assembly intermediates

Authors: Lydia Hadjeras , Marie Bouvier , Isabelle Canal , Leonora Poljak , Quentin Morin-Ogier , Carine Froment , Odile Burlet-Schlitz , Lina Hamouche , Laurence Girbal , Muriel Cocaign-Bousquet , Agamemnon J. Carpousis

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RNA processing and degradation shape the transcriptome by generating stable molecules that are necessary for translation (rRNA and tRNA) and by facilitating the turnover of mRNA, which is necessary for the posttranscriptional control of gene expression. In bacteria and the plant chloroplast, RNA degradosomes are multienzyme complexes that process and degrade RNA. In many bacterial species, the endoribonuclease RNase E is the central component of the RNA degradosome. RNase E-based RNA degradosomes are inner membrane proteins in a large family of gram-negative bacteria (β- and γ-Proteobacteria). Until now, the reason for membrane localization was not understood. Here, we show that a mutant strain of Escherichia coli, in which the RNA degradosome is localized to the interior of the cell, has high levels of 20S and 40S particles that are defective intermediates in ribosome assembly. These particles have aberrant protein composition and contain rRNA precursors that have been cleaved by RNase E. After RNase E cleavage, rRNA fragments are degraded to nucleotides by exoribonucleases. In vitro, rRNA in intact ribosomes is resistant to RNase E cleavage, whereas protein-free rRNA is readily degraded. We conclude that RNA degradosomes in the nucleoid of the mutant strain interfere with cotranscriptional ribosome assembly. We propose that membrane-attached RNA degradosomes in wild-type cells control the quality of ribosome assembly after intermediates are released from the nucleoid. That is, the compact structure of mature ribosomes protects rRNA against cleavage by RNase E. Turnover of a proportion of intermediates in ribosome assembly explains slow growth of the mutant strain. Competition between mRNA and rRNA degradation could be the cause of slower mRNA degradation in the mutant strain. We conclude that attachment of the RNA degradosome to the bacterial inner cytoplasmic membrane prevents wasteful degradation of rRNA precursors, thus explaining the reason for conservation of membrane-attached RNA degradosomes throughout the β- and γ-Proteobacteria.

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Some Aspects of Overturning Mechanisms of Pile Driving Machine on Soft Foundation

Authors: Shouji Toma , Wai Fah Chen

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Many overturning accidents of cranes and pile driving machines have occurred in recent years. Overturning mechanisms of those accidents have been studied by the author, et al., based on theory of structural stability by using the rotational spring-rigid bar model. A summary of the theoretical study is first described in this paper including the concept of the structural stability, the category of the overturning mechanisms and the preliminary model test. Then, using the same structural model, the computer structural analysis, in which software TDAPIII (Time-domain Dynamic Analysis Program in 3-dimension) is used, is explained in order to compare with the previous theoretical study. Consequently, the computer analysis confirms a good coincidence with the theoretical results. Further, by the computer analysis, the effect of the rotational stiffness of the foundation and the eigenvalue analysis to find the critical load are studied. In general, the computer analysis is a more practical tool to study the overturning mechanism than the theoretical analysis which is based on the equilibrium equations. The computer analysis would be possible to consider the material non-linearity of the foundation and 3-dimensional effect. In order to avoid this kind of overturning accident in the future, the results in this paper will give useful information to study further in details.

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