Osmotic compaction of supercoiled DNA into a bacterial nucleoid

A possible role for L24 of Bacillus subtilis in nucleoid organization and segregation

The condensation of DNA in bacterial nucleoids during cell cycle is a complex and dynamic process. Proteins displaying the physico-chemical properties of histones are known to contribute to this process. During a search for B. subtilis nucleoid associated proteins, HBsu and L24 were identified as the most abundant proteins in nucleoid containing fractions. Purified L24 binds and condenses DNA in vitro. In this paper we describe immunofluorescence studies that demonstrated that L24 is located at the poles of the nucleoids in exponentially growing cells. In contrast, the protein is dispersed in the cytoplasm during stationary phase. Moreover, overexpression of the rplX gene encoding L24 disrupts nucleoid segregation and positioning.

A Limited Loss of DNA Compaction Accompanying the Release of Cytoplasm from Cells of Escherichia coli

The DNA of bacteria is compacted into nucleoids. We have lysed cells of Escherichia coli under conditions in which the cell envelope is retained. The extent of DNA compaction was determined by light microscopy, comparing DAPI fluorescence and phase contrast images. The release of cytoplasm upon lysis allowed the nucleoidal DNA to expand to fill the residual cell boundaries, supporting the role of cytoplasmic crowding in nucleoid compaction. The addition of polylysine allowed lysis with retention of DNA compaction. Furthermore, chloramphenicol treatment of cells resulted in nucleoids which were more resistant to decompaction upon lysis.

Delayed nucleoid segregation in Escherichia coli

To study the role of cell division in the process of nucleoid segregation, we measured the DNA content of individual nucleoids in isogenic Escherichia coli cell division mutants by image cytometry. In pbpB(Ts) and ftsZ strains growing as filaments at 42 degrees C, nucleoids contained, on average, more than two chromosome equivalents compared with 1.6 in wild-type cells. Because similar results were obtained with a pbpB recA strain, the increased DNA content cannot be ascribed to the occurrence of chromosome dimers. From the determination of the amount of DNA per cell and per individual nucleoid after rifampicin inhibition, we estimated the C and D periods (duration of a round of replication and time between termination and cell division respectively), as well as the D' period (time between termination and nucleoid separation). Compared with the parent strain and in contrast to ftsQ, ftsA and ftsZ mutants, pbpB(Ts) cells growing at the permissive temperature (28 degrees C) showed a long D' period (42 min versus 18 min in the parent) indicative of an extended segregation time. The results indicate that a defective cell division protein such as PbpB not only affects the division process but also plays a role in the last stage of DNA segregation. We propose that PbpB is involved in the assembly of the divisome and that this structure enhances nucleoid segregation.

Electrostatic-undulatory theory of plectonemically supercoiled DNA

We present an analytical calculation of the electrostatic interaction in a plectonemic supercoil within the Poisson-Boltzmann approximation. Undulations of the supercoil strands arising from thermal motion couple nonlinearly with the electrostatic interaction, giving rise to a strong enhancement of the bare interaction. In the limit of fairly tight winding, the free energy of a plectonemic supercoil may be split into an elastic contribution containing the bending and torsional energies and an electrostatic-undulatory free energy. The total free energy of the supercoil is minimized according to an iterative scheme, which utilizes the special symmetry inherent in the usual elastic free energy of the plectoneme. The superhelical radius, opening angle, and undulation amplitudes in the radius and pitch are obtained as a function of the specific linking difference and the concentration of monovalent salt. Our results compare favorably with the experimental values for these parameters of Boles et al. (1990. J. Mol. Biol. 213:931-951). In particular, we confirm the experimental observation that the writhe is a virtually constant fraction of the excess linking number over a wide range of superhelical densities. Another important prediction is the ionic strength dependence of the plectonemic parameters, which is in reasonable agreement with the results from computer simulations.