Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 90

Proteins called condensins, which help to condense DNA in the cell, were first discovered in eukaryotes. They are also known as SMC proteins for structural maintenance of chromosome. Condensins are long, dumbbell-shaped proteins with globular domains at the ends and a long coiled-coil region holding them together. The long coiledcoil region has a hinge so that it can fold back on itself and the two globular domains can bind together. Condensins work with partner proteins to help condense DNA into a higher-order structure. While still somewhat controversial, bacterial condensins may encircle DNA to gather regions on the DNA together that can subsequently be held together by a variety of other sequence-nonspecific DNA-binding proteins discussed elsewhere, such as HU, IHF, H-NS, and Fis.

Figure 1.19 Model of the way in which chromosome decatenation by topoisomerase IV (Topo IV) 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 yellow circle, and double-stranded DNA is shown as a single line for simplicity.

The condensin of E. coli, called MukB, was found because mutations in its gene interfere with chromosome segregation. MukB was suspected of condensing the DNA because the protein and supercoiling of the DNA can compensate for each other in allowing proper segregation of the daughter chromosomes into daughter cells. As indicated above, supercoiling can lead to the formation of precatenanes and catenanes that are removed by Topo IV. The removal of precatenanes and catenanes also appears to be regulated with the condensation of chromosomes by an interaction between one of the Topo IV subunits and MukB (Figure 1.19) (see Hayama and Marians, Suggested Reading). MukB interacts with DNA through association with the partner proteins MukF and MukE. B. subtilis also has a condensin, which is more similar in amino acid sequence to the eukaryotic condensins and so was also named SMC protein (see Britton et al., Suggested Reading). It also has partner proteins named ScpA and ScpB. In B. subtilis, the link between chromosome condensation and partitioning is becoming clearer with the finding that proteins that directly recognize the region around the origin and are involved in partitioning are able to recruit the condensin SMC in this organism (see Thanbichler, Suggested Reading).