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      MukB colocalizes with the oriC region and is required for organization of the two Escherichia coli chromosome arms into separate cell halves

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          Abstract

          The circular Escherichia coli chromosome is organized by bidirectional replication into two equal left and right arms (replichores). Each arm occupies a separate cell half, with the origin of replication ( oriC) at mid-cell. E. coli MukBEF belongs to the ubiquitous family of SMC protein complexes that play key roles in chromosome organization and processing. In mukBEF mutants, viability is restricted to low temperature with production of anucleate cells, reflecting chromosome segregation defects. We show that in mukB mutant cells, the two chromosome arms do not separate into distinct cell halves, but extend from pole to pole with the oriC region located at the old pole. Mutations in topA, encoding topoisomerase I, do not suppress the aberrant positioning of chromosomal loci in mukB cells, despite suppressing the temperature-sensitivity and production of anucleate cells. Furthermore, we show that MukB and the oriC region generally colocalize throughout the cell cycle, even when oriC localization is aberrant. We propose that MukBEF initiates the normal bidirectional organization of the chromosome from the oriC region.

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          Cohesin relocation from sites of chromosomal loading to places of convergent transcription.

          Sister chromatids, the products of eukaryotic DNA replication, are held together by the chromosomal cohesin complex after their synthesis. This allows the spindle in mitosis to recognize pairs of replication products for segregation into opposite directions. Cohesin forms large protein rings that may bind DNA strands by encircling them, but the characterization of cohesin binding to chromosomes in vivo has remained vague. We have performed high resolution analysis of cohesin association along budding yeast chromosomes III-VI. Cohesin localizes almost exclusively between genes that are transcribed in converging directions. We find that active transcription positions cohesin at these sites, not the underlying DNA sequence. Cohesin is initially loaded onto chromosomes at separate places, marked by the Scc2/Scc4 cohesin loading complex, from where it appears to slide to its more permanent locations. But even after sister chromatid cohesion is established, changes in transcription lead to repositioning of cohesin. Thus the sites of cohesin binding and therefore probably sister chromatid cohesion, a key architectural feature of mitotic chromosomes, display surprising flexibility. Cohesin localization to places of convergent transcription is conserved in fission yeast, suggesting that it is a common feature of eukaryotic chromosomes.
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            The structure and function of SMC and kleisin complexes.

            Protein complexes consisting of structural maintenance of chromosomes (SMC) and kleisin subunits are crucial for the faithful segregation of chromosomes during cell proliferation in prokaryotes and eukaryotes. Two of the best-studied SMC complexes are cohesin and condensin. Cohesin is required to hold sister chromatids together, which allows their bio-orientation on the mitotic spindle. Cleavage of cohesin's kleisin subunit by the separase protease then triggers the movement of sister chromatids into opposite halves of the cell during anaphase. Condensin is required to organize mitotic chromosomes into coherent structures that prevent them from getting tangled up during segregation. Here we describe the discovery of SMC complexes and discuss recent advances in determining how members of this ancient protein family may function at a mechanistic level.
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              Entropy-driven spatial organization of highly confined polymers: lessons for the bacterial chromosome.

              Despite recent progress in visualization experiments, the mechanism underlying chromosome segregation in bacteria still remains elusive. Here we address a basic physical issue associated with bacterial chromosome segregation, namely the spatial organization of highly confined, self-avoiding polymers (of nontrivial topology) in a rod-shaped cell-like geometry. Through computer simulations, we present evidence that, under strong confinement conditions, topologically distinct domains of a polymer complex effectively repel each other to maximize their conformational entropy, suggesting that duplicated circular chromosomes could partition spontaneously. This mechanism not only is able to account for the spatial separation per se but also captures the major features of the spatiotemporal organization of the duplicating chromosomes observed in Escherichia coli and Caulobacter crescentus.
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                Author and article information

                Journal
                Mol Microbiol
                mmi
                Molecular Microbiology
                Blackwell Publishing Ltd
                0950-382X
                1365-2958
                September 2007
                : 65
                : 6
                : 1485-1492
                Affiliations
                Department of Biochemistry, University of Oxford Oxford OX1 3QU, UK
                Author notes
                *E-mail sherratt@ 123456bioch.ox.ac.uk ; Tel. +44 (0) 1865 275 296; Fax +44 (0) 1865 275 297; Email possoz@ 123456cgm.cnrs-gif.fr ; Tel. +33 (0) 1 69 82 31 70; Fax +33 (0) 1 69 82 31 60.
                [†]

                Present address: CGM Bât. 26, CNRS, Allée de la terrasse, 91198 Gif sur Yvette, France.

                Re-use of this article is permitted in accordance with the Creative Commons Deed, Attribution 2.5, which does not permit commercial exploitation.

                Article
                10.1111/j.1365-2958.2007.05881.x
                2169520
                17824928
                62035e2a-3b20-4013-9940-70b4c0424269
                © 2007 The Authors; Journal compilation © 2007 Blackwell Publishing Ltd
                History
                : 17 July 2007
                Categories
                Research Articles

                Microbiology & Virology
                Microbiology & Virology

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