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Liniowe plazmidy bakteryjne

100%

Łobocka M.

Kosmos

2002

vol. 51

issue 3

283-296

Summary Completion of linear DNA replication requires a way to restore the original sequence and structure of linear DNA ends which can not be fully replicated by conventional DNA polymerases. In bacteria, the end replication problem has been circumvented through the use of circular plasmids and chromosomes. However linear bacterial plasmids and chromosomes have also been isolated. Their ends, commonly known as prokaryotic telomers, differ in structure from the ends of eukaryotic chromosomes and, during replication, become restored to their original form by a different mechanism. Two kinds of linear plasmids have been isolated: plasmids with covalently closed hairpin ends, and plasmids with invertron ends, which contain proteins bound to their 5' termini. The latter constitute the larger group and are commonly found in actinomycetous bacteria. They are usually conjugative and confer advantageous phenotypes. Plasmids with covalently closed ends are common in spirochetes of the genus Borrelia. A model plasmid of this group is prophage N15 of Escherichia coli, which exists in lysogens as a linear DNA molecule. The major difference between circular and linear plasmids is the presence in the latter of linear ends and proteins that specifically recognize those ends and are able to restore plasmid linearity during or after replication. Complete replication of invertron telomers depends on their 5' end-associated proteins but its mechanism is still unclear. Plasmids with covalently closed ends are completely replicated from an internal origin to form circular dimeric molecules that can be observed as replication intermediates. Further processing of the intermediates depends on telomere resolution, a DNA breakage and reunion reaction, in which opposite strands of replicated telomeres are cleaved and rejoined to form covalently closed ends of two progeny molecules.

Partycja niskokopiowych plazmidów

51%

Łobocka M., Bugajska O., Dobruk A.

Kosmos

2002

vol. 51

issue 3

305-318

Summary Partitioning of low-copy number plasmids at cell division resembles mitosis in that prior to cell division paired plasmid molecules are separated from each other and actively moved apart from the center of the mother cell into positions corresponding to the centers of future daughters. Two plasmid proteins and a cis-acting site in plasmid DNA, an analog of eukaryotic centromere, are essential for partitioning to occur. Typically, partition proteins are encoded within an operon whose expression is regulated by one or both of its products. The product of the first gene of the partition operon (ParA, SopA or ParM) is an ATPase, the product of the second gene (ParB, SopB or ParR) is a centromere-binding protein. Partitioning ATPases belong to the Walker-type or actin-type ATPase families. They interact with their target centromeric complexes indirectly, via plasmid specific centromere- binding proteins. Two types of partitioning ATPases may provide two different ways of plasmid translocation within a cell. The movement of plasmid Rl, which encodes the actin-type ParM ATPase, is associated with extension of ParM filaments at their polar end and depolymerization at the opposite end. Extension might possibly occur by insertion of new ParM molecules between the centromere-bound ParR proteins of paired Rl plasmids and ParM molecules bound to ParR, thus pushing the plasmids towards the opposite cell poles. The role of Walker-type ATPases in plasmid translocation is not clear. In their structure they resemble some ion pump proteins. They may either participate in plasmid movement by themselves or connect plasmids to unknown partition machinery of a host. Some of them can oscillate between cellular poles, reminiscent of the behavior of related bacterial proteins, MinD, that specify the sites of septum placement. Partitioning mechanisms in bacteria are evolutionarily conserved and of universal occurrence. Plasmid partition operons with centromeric sites can function as gene cassettes able to stabilize other unstable low-copy number plasmids. Certain bacterial chromosomes encode homologs of plasmid partition genes essential for chromosome partitioning.

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2 Kosmos

2 2002

2 Łobocka M.

1 Bugajska O.

1 Dobruk A.

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Liniowe plazmidy bakteryjne

Partycja niskokopiowych plazmidów