Structure And Stabilization Of The Bacterial Nucleoid

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The DNA of bacteria is localized in a few compact bodies per cell known as nucleoids. Control of compaction offers a virtually unexplored means of interacting with and potentially controlling bacterial growth. We have previously developed quantitative procedures to assay factors that are involved in nucleoid compaction in Escherichia coli. One of these factors, known for 30 years but never identified, is characterized by its RNase-sensitivity. By use of the new assay procedures, this RNase-sensitive component has been identified as the ribosomal link in cotranslational insertion linkages between the nucleoidal DNA and the cell envelope. The rounded nucleoids ("cm-nucleoids)that are formed upon exposure of E. coli to chloramphenicol, as well as under several other conditions, are of particular interest. Cm-nucleoids are extraordinarily stable to denaturation and appear to be formed by a relatively direct and efficiently reversible conversion from normal nucleoids, offering potential insights into the unknown structure of the normal nucleoid. Although a hollow spherical model had been repeatedly presented for cm-nucleoids by another laboratory, that model is unable to account for the light microscope appearance of a significant fraction of cm-nucleoids. A new type of curved toroidal structure is proposed for cm-nucleoids which can rationalize the previously discordant observations.