Snyder and Champness Molecular Genetics of Bacteria. Tina M. Henkin

Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin


Скачать книгу
After aligning the dif sites, the FtsK protein also interacts with the XerCD enzyme, allowing it to resolve the dimer chromosomes at the dif sites. (C) The coordinated activities of the dimer resolution system and FtsK lead to monomer chromosomes that are capable of full segregation to daughter cells. From Camara JE, Crooke E, in Higgins NP (ed), The Bacterial Chromosome (ASM Press, Washington, DC, 2005).

      Homologs of FtsK are widespread, and a homologous protein called SftA appears to carry out similar functions in B. subtilis (see Biller and Burkholder, Suggested Reading). Some bacteria have more than one FtsK-like protein, presumably for other specialized tasks. In the case of B. subtilis, another FtsK homolog, SpoIIIE, is responsible for translocating the final third of the chromosome into spores during spore development so the spore will get a complete copy of the chromosome (see chapter 12). How the roles of FtsK, SpoIIIE, and SftA differ and how the multiple members are involved in various DNA processing events in a single cell remain active areas of research with many questions still to be answered.

      Interestingly, instead of XerC and XerD, Streptococcus and Lactococcus species have a system more closely related to bacteriophage integration systems that uses a single protein called XerS to carry out the same function (see Le Bourgeois et al., Suggested Reading). Archaea also seem to use a single protein (in this case called XerA) to resolve dimer chromosome at a cis-acting dif site (see Cortez et al., Suggested Reading). The regulation of dimer chromosome resolution in these systems and any involvement of FtsK-like proteins are unclear.

Schematic illustration of model of the way in which unwinding of the template DNA strands which can cause twists that diffuse across the replication complex and twist the new DNA strands. (A) The replication machinery must open the double-stranded DNA to copy the template strands. (B) The template DNA strands introduces twists called positive supercoils ahead of the replication fork. (C) Some of the torsion that is generated ahead of the replication fork can be relieved by rotation of the replication complex itself. The torsional stress can spread behind the fork and intertwine the new copies of the chromosome. (D) Precatenanes result in links in the daughter chromosomes called catenanes that must be unlinked for the chromosomes to separate into daughter cells.

      CONDENSATION

      Bacterial cells have an important mechanism to help manage chromosomes, which is to condense them after DNA replication. If the daughter chromosomes are condensed, they do not overlap as much in the cell and so are less apt to become interlinked. Condensation of chromosomes prior to mitosis has been known for a long time to occur in eukaryotic cells, where the chromosomes are only clearly visible just before mitosis. We now know that bacteria also condense their daughter DNAs to make them easier to manage, even though it is more difficult to visualize the condensation of bacterial chromosomes because of their smaller size.

      Condensins

Schematic illustration of model of the way in which chromosome decatenation by topoisomerase IV and is coordinated with chromosome condensation by MukB and chromosome translocation with FtsK. The two daughter chromosomes that have been replicated are indicated by light and dark blue lines, and the unreplicated portion of the chromosome is shown in gray. Unwinding of the template DNA strands that is associated with DNA replication twists the newly replicated strands of DNA, forming precatenanes that go on to become catenanes if not unlinked by the action of Topo IV. Topo IV can interact with the chromosome condensation protein MukB to remove catenanes before DNA is condensed. Topo IV also interacts with the FtsK translocase to coordinate decatenation with chromosome segregation. The replisome is shown as a circle, and double-stranded DNA is shown as a single line for simplicity.
Скачать книгу