Plasmid partition system

Partition system ensures stable transmission of plasmids during bacterial cell division. The main maintenance function of plasmids is played by its independent replication system which control the number of copy of a plasmid in a cell. The higher the copy number is, the more likely the two daughter cells will contain the plasmid where each molecule of plasmid diffuses randomly. Hence, the probability of having plasmid-less cell is 2^1-N, where N is the number of copy. For instance, if a plasmid has 2 copies of it in a cell, there is 50% of 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 high probability of loss after few generation. In addition to controlled-replication systems, bacterial plasmids use different maintenance strategies, such as multimer resolution systems, post-segregational killing systems or addiction module, and partition systems. This latter will be discussed here.

General properties of partition systems

Plasmid copies are paired around a centromere-like site and then separated in the two daughter cell. Partition systems involve three elements :

• centromere-like DNA site
• Centromere binding protein (CBP)
• Motor protein

These three elements are encoded on the plasmid DNA and are organized as an operon.

Centromer-like DNA site, also known as par 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 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 : Walker type P-loop ATPase
• Type II : Actin-like ATPase
• Type III : tubulin-like GTPase
• Type IV : No NTPase

Name of the different elements in different types

Type	Motor protein	Centromere binding protein	Centromere-like binding site	Other proteins
Type I	ParA	ParB	parS
Type II	ParM	ParR	parC
Type III	TubZ	TubR	tubS	TubY
Type IV	StbB*	StbA	stbDRs	StbC

^* StbB is an ATPase but not implied in the partitioning.