Plasmid partition system

A plasmid partition system is a mechanism that ensures the stable inheritance of plasmids during bacterial cell division. Each plasmid has its independent replication system which controls the number of copies of the plasmid in a cell. The higher the copy number is, the more likely the two daughter cells will contain the plasmid. Generally, each molecule of plasmid diffuses randomly, so the probability of having a plasmid-less daughter cell is 21−N, where N is the number of copies. For instance, if there are 2 copies of a plasmid in a cell, there is a 50% chance of having one plasmid-less daughter cell. However, high-copy number plasmids have a cost for the hosting cell. This metabolic burden is lower for low-copy plasmids, but those have a higher probability of plasmid loss after a few generations. To control vertical transmission of plasmids, in addition to controlled-replication systems, bacterial plasmids use different maintenance strategies, such as multimer resolution systems, post-segregational killing systems (addiction modules), and partition systems.
General properties of partition systems.
Plasmid copies are paired around a centromere-like site and then separated in the two daughter cells. Partition systems involve three elements, organized in an auto-regulated operon:
The centromere-like DNA site is required in "cis" for plasmid stability. It often contains one or more inverted repeats which are recognized by multiple CBPs. This forms a nucleoprotein complex termed the partition complex. This complex recruits the motor protein, which is a nucleotide triphosphatase (NTPase). The NTPase uses energy from NTP binding and hydrolysis to directly or indirectly move and attach plasmids to specific host location (e.g. opposite bacterial cell poles).
The partition systems are divided in four types, based primarily on the type of NTPases:
Type I partition system.
This system is also used by most bacteria for chromosome segregation. 
Type I partition systems are composed of an ATPase which contains Walker motifs and a CBP which is structurally distinct in type Ia and Ib. ATPases and CBP from type Ia are longer than the ones from type Ib, but both CBPs contain an arginine finger in their N-terminal part.
ParA proteins from different plasmids and bacterial species show 25 to 30% of sequence identity to the protein ParA of the plasmid P1.
The partition of type I system uses a "diffusion-ratchet" mechanism. This mechanism works as follows:
There are likely to be differences in the details of type I mechanisms.
Type 1 partition has been mathematically modelled with variations in the mechanism described above.
Type Ia.
The CBP of this type consists in three domains:
Type Ib.
The CBP of this type, also known as "parG" is composed of:
For this type, the "parS" site is called "parC".
Type II partition system.
This system is the best understood of the plasmid partition system.
It is composed of an actin-like ATPAse, ParM, and a CBP called ParR. The centromere like site, "parC" contains two sets of five 11 base pair direct repeats separated by the "parMR" promoter.
The amino-acid sequence identity can go down to 15% between ParM and other actin-like ATPase.
The mechanism of partition involved here is a pushing mechanism:
The filament of ParM is regulated by the polymerization allowed by the presence the partition complex (ParR-"parC"), and by the depolymerization controlled by the ATPase activity of ParM.
Type III partition system.
The type III partition system is the most recently discovered partition system. It is composed of tubulin-like GTPase termed TubZ, and the CBP is termed TubR.
Amino-acid sequence identity can go down to 21% for TubZ proteins.
The mechanism is similar to a treadmill mechanism:
The net result being transport of partition complex to the cell pole.
Other partition systems.
R388 partition system.
The partition system of the plasmid R388 has been found within the "stb" operon. This operon is composed of three genes, "stbA", "stbB" and "stbC".
The StbA-"stbDRs" complex may be used to pair plasmid the host chromosome, using indirectly the bacterial partitioning system.
StbA and StbB have opposite but connected effect related to conjugation.
This system has been proposed to be the type IV partition system. It is thought to be a derivative of the type I partition system, given the similar operon organization.
This system represents the first evidence for a mechanistic interplay between plasmid segregation and conjugation processes.
pSK1 partition system (reviewed in ).
pSK1 is a plasmid from Staphylococcus aureus. This plasmid has a partition system determined by a single gene, "par", previously known as "orf245". This gene does not effect the plasmid copy number nor the grow rate (excluding its implication in a post-segregational killing system). A centromere-like binding sequence is present upstream of the "par" gene, and is composed of seven direct repeats and one inverted repeat.