Speculations On Chromosome Packaging Dynamics

In the past two decades, our understanding of bacterial chromosomes has advanced remarkably along two fronts. First, a combination of genetics and biochemistry has taught us a great deal about the proteins that manipulate DNA. Second, we now have complete genomic sequences for many bacteria. These sequences indicate that bacterial genomes are quite flexible with respect to gene order. However, few experiments bear on how the long chromosomal DNA molecule is compacted to fit inside a cell. Here, we offer a speculative scheme that may help form a framework for understanding future discoveries.

One key concept is that cytoplasmic proteins and other large cytoplasmic molecules are at such high concentration that they compact DNA through macromolecular crowding. This idea, which has recently been refined by Zimmerman, requires no specific DNA compacting proteins, and so it accommodates the apparent absence of nucleosome-like particles in eubacteria. We envision that chromosomal activities involving bulky protein complexes occur at the edges of the nucleoid (Fig. 21.5). For example, the replication apparatus, which is likely to be attached to a multienzyme complex that supplies deoxyribo-nucleoside triphosphates, is probably situated at the edge of the compacted portion of the chromosome, especially if replication proteins are bound to the cell membrane. Likewise, transcription, which in bacteria is coupled to translation, also probably occurs on DNA emerging from the compacted mass of nucleoid DNA because ribosomes are seen only outside the nucleoid (extrachromosomal localization is especially likely when transcription-translation complexes are bound to the cell membrane via nascent membrane proteins). Consistent with this idea, pulse-labeled nascent RNA is preferentially located at the nucleoid border, as is topoisomerase I (as noted previously, topoisomerase I may serve as a cytological marker for transcription since it is probably localized behind transcription complexes to prevent excess negative supercoils from accumulating).

If the replication and transcription-translation machinery are located on the surface of the nucleoid,

FIGURE 21.5 Overview of bacterial chromosome structure. The figure shows a schematic representation of a bacterial cell and its chromosomal DNA (nucleoid). The replication apparatus is located at the edge of the nucleoid at a mid-cell position. Two replication forks (arrowheads) are shown in close proximity. DNA is thought to thread through a stationary replication apparatus. The funnel-like structure represents a multienzyme complex responsible for synthesis of deoxyribonucleoside triphosphates (dNTPs) from ribonucleoside diphosphates (R). In the enlargement of the replication apparatus the dashed lines outside the circle represent the connections between the DNA strands masked by the large amount of DNA in the cell. The polar distribution of oriC regions is indicated. Macromolecular crowding (arrows) contributes to DNA condensation, with additional packing occurring from protein-induced DNA bending (B) at many points on the chromosome (only one bending point is labeled). Two examples of coupled transcription-translation are shown to occur at the edge of the nucleoid. In the lower left, nascent protein is bound to the cell membrane, drawing a region of DNA out of the compact part of the nucleoid. To provide transcriptional access to all regions of the genome, the interior and exterior regions of the nucleoid are assumed to exchange rapidly.

FIGURE 21.5 Overview of bacterial chromosome structure. The figure shows a schematic representation of a bacterial cell and its chromosomal DNA (nucleoid). The replication apparatus is located at the edge of the nucleoid at a mid-cell position. Two replication forks (arrowheads) are shown in close proximity. DNA is thought to thread through a stationary replication apparatus. The funnel-like structure represents a multienzyme complex responsible for synthesis of deoxyribonucleoside triphosphates (dNTPs) from ribonucleoside diphosphates (R). In the enlargement of the replication apparatus the dashed lines outside the circle represent the connections between the DNA strands masked by the large amount of DNA in the cell. The polar distribution of oriC regions is indicated. Macromolecular crowding (arrows) contributes to DNA condensation, with additional packing occurring from protein-induced DNA bending (B) at many points on the chromosome (only one bending point is labeled). Two examples of coupled transcription-translation are shown to occur at the edge of the nucleoid. In the lower left, nascent protein is bound to the cell membrane, drawing a region of DNA out of the compact part of the nucleoid. To provide transcriptional access to all regions of the genome, the interior and exterior regions of the nucleoid are assumed to exchange rapidly.

DNA movement must occur to allow access to all nucleotide sequences. Replication-based movement probably occurs by DNA threading through stationary replication forks. Such movement would not be sufficient for transcriptional access to the whole genome since some genes can be induced when DNA replication is not occurring. Perhaps compacted DNA is sufficiently fluid that genes frequently pass from interior to exterior. At any given moment, in some fraction of the cell population each gene may be at the surface of the nucleoid and available for transcription. Capture of a gene by the transcription-translation apparatus would hold that gene on the surface. During induction of gene expression, the fraction of cells in which a particular gene is captured would increase until most of the cells express that gene. For the chromosome as a whole, many genes would be expressing protein during active growth, and therefore many regions would be held outside the nucleoid core by the transcription-translation apparatus. Kellenberger suggested that such activity explains why the nucleoid appears more compact when protein synthesis is experimentally interrupted. The idea of gene capture for expression requires that the replication apparatus be strong enough to pull the DNA through itself even when genes are bound to ribosomes via mRNA and to the cell membrane via nascent proteins still attached to ribosomes. A fixed RNA polymerase must also pull DNA.

Capture of the oriC region by the replication apparatus might be similar to gene capture for transcription. With the fluid chromosome hypothesis, some replication proteins would assemble at mid-cell, whereas others would assemble with the centromerelike DNA elements and SpoOJ (in B. subtilis) at the poles and dislodge oriC from its polar connection. Once liberated, a mobile oriC would be captured by the replication apparatus at mid-cell. As oriC and nearby regions are drawn through the replication forks and replicated, new binding sites for a chromosome partition protein (SpoOJ) would be created. Once these sites were filled, the two daughter oriC regions might pair through SpoOJ-SpoOJ interactions and return to the polar position. Other proteins would later disrupt the SpoOJ-SpoOJ interactions as the new SpoOJ-oriC complex is moved to the other pole of the nucleoid.

Movement of DNA must generate tangles, just as loops in fishing line snarl when reels are not carefully attended. The decatenating activities of the type II topo-isomerases, DNA gyrase and topoisomerase IV, are available to remove the tangles, explaining in part why these two enzymes are found at many spots on the chromosome. The movement of the daughter chromosomes to opposite cell poles would provide the directionality needed by the topoisomerases to untangle the loops.

The abundant DNA bending proteins, such as HU, IHF, and H-NS, probably facilitate chromosome compaction in a dynamic manner by rapidly exchanging between DNA-bound and unbound states. Likewise, the topoisomerases probably respond rapidly to local perturbations in supercoiling to maintain the proper level of supercoiling throughout the chromosome.

Fixed topological domains are not easily accomo-dated by a fluid model for bacterial chromosome structure, since they require specialized, pulley-like structures to allow the DNA movement needed for replication. Transient domains could be generated by recombination intermediates, provided that recombination occurs often enough to maintain approximately 50 domains per genome.

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