13. Partioning involves membrane attachment and (possibly) a motor

12.13 Partioning involves membrane attachment and (possibly) a motor


Partitioning is the process by which the two daughter chromosomes find themselves on either side of the position at which the septum forms. Two types of event are required for proper partitioning:



  • The two daughter chromosomes must be released from one another so that they can segregate following termination. This requires disentangling of DNA regions that are coiled around each other in the vicinity of the terminus. Most mutations affecting partitioning map in genes coding for topoisomerases Venzymes with the ability to pass DNA strands through one another. The mutations prevent the daughter chromosomes from segregating, with the result that the DNA is located in a single large mass at midcell. Septum formation then releases an anucleate cell and a cell containing both daughter chromosomes. This tells us that the bacterium must be able to disentangle its chromosomes topologically in order to be able to segregate them into different daughter cells.
  • Mutations that affect the partition process itself are rare. We expect to find two classes. cis-acting mutations should occur in DNA sequences that are the targets for the partition process. trans-acting mutations should occur in genes that code for the protein(s) that cause segregation, which could include proteins that bind to DNA or activities that control the locations on the envelope to which DNA might be attached. (Both types of mutation have been found in the systems responsible for partitioning plasmids Xsee later Xbut only trans-acting functions have been found in the bacterial chromosome; for review see Hiraga, 1992; Wake and Errington, 1995; Wake and Errington, 1995)



Figure 12.26 Attachment of bacterial DNA to the membrane could provide a mechanism for segregation.

The original form of the model for chromosome segregation shown in Figure 12.26 suggested that the envelope grows by insertion of material between the attachment sites of the two chromosomes, thus pushing them apart. But in fact the cell wall and membrane grow heterogeneously over the whole cell surface. Furthermore, the replicated chromosomes are capable of abrupt movements to their final positions at ¼ and ¾ cell length. If protein synthesis is inhibited before the termination of replication, the chromosomes fail to segregate and remain close to the midcell position. But when protein synthesis is allowed to resume, the chromosomes move to the quarter positions in the absence of any further envelope elongation. This suggests that an active process, requiring protein synthesis, may move the chromosomes to specific locations.


Segregation is interrupted by mutations of the muk class, which give rise to anucleate progeny at a much increased frequency: both daughter chromosomes remain on the same side of the septum instead of segregating. Mutations in the muk genes are not lethal, and may identify components of the apparatus that segregates the chromosomes. The gene mukA is identical with the gene for a known outer membrane protein (tolC), whose product could be involved with attaching the chromosome to the envelope. The gene mukB codes for a large (180 kD) globular protein, which has some sequence relationship to the mechanochemical enzyme dynamin, which provides a "motor" for microtubule Vassociated objects. This suggests the possibility that MukB is a motor that physically moves the chromosome relative to the envelope.


There have been suspicions for years that a physical link exists between bacterial DNA and the membrane, but the evidence remains indirect. Bacterial DNA can be found in membrane fractions, which tend to be enriched in genetic markers near the origin, the replication fork, and the terminus. The proteins present in these membrane fractions may be affected by mutations that interfere with the initiation of replication. The growth site could be a structure on the membrane to which the origin must be attached for initiation (Jacob et al., 1966).




Figure 12.30 A common segregation system consists of genes parA and parB and the target site parS.

Functions involved in partition were first identified in plasmids. The components of a common system are summarized in Figure 12.30. Typically there are two trans-acting loci and a cis-acting element located just downstream of the two genes. ParA is an ATPase. It binds to ParB, which binds to the parS site on DNA. Deletions of any of the three loci prevent proper partition of the plasmid. It seems likely that the ParA VParB oligomer binds to some cellular structure, so that parS effectively behaves as a centromere.




Figure 9.23 Sporulation involves successive changes in the sigma factors that control the initiation specificity of RNA polymerase. The cascades in the forespore (left) and the mother cell (right) are related by signals passed across the septum (indicated by horizontal arrows).

Proteins related to ParA and ParB are found in several bacteria. In B. subtilis, they are called Soj and SpoOJ, respectively. Mutations in these loci prevent sporulation, because of a failure to segregate one daughter chromosome into the forespore (see Figure 9.23). In sporulating cells, SpoOJ localizes at the pole and may be responsible for localizing the origin there. SpoOJ binds to a sequence that is present in multiple copies, dispersed over ~20% of the chromosome in the vicinity of the origin. It is possible that SpoOJ binds both old and newly synthesized origins, maintaining a status equivalent to chromosome pairing, until the chromosomes are segregated to the opposite poles. In C. crescentus, ParA and ParB localize to the poles of the bacterium, and ParB binds sequences close to the origin, thus localizing the origin to the pole. These results suggest that a specific apparatus is responsible for localizing the origin to the pole. The next stage of the analysis will be to identify the cellular components with which this apparatus interacts (Mohl and Gober, 1997).


This section updated 5-8-2000




Reviews
Hiraga, S. (1992). Chromosome and plasmid partition in E. coli. Ann. Rev. Biochem 61, 283-306.
Wake, R. G. and Errington, J. (1995). Chromosome partitioning in bacteria. Ann. Rev. Genet. 29, 41-67.
Wake, R. G. and Errington, J. (1995). Chromosome partitioning in bacteria. Ann. Rev. Genet. 29, 41-67.

Research
Jacob, F., Ryter, A., and Cuzin, F. (1966). On the association between DNA and the membrane in bacteria. Proc R Soc Lond B Biol Sci 164, 267-348.
Mohl, D. A. and Gober, J. W. (1997). Cell cycle-dependent polar localization of chromosome partitioning proteins of C crescentus. Cell 88, 675-684.



Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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