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.
* StbA protein is a DNA-binding protein (identical to [https://www.uniprot.org/uniprot/P11904 ParM]) and is strictly required for the stability and intracellular positioning of plasmid R388 in E. coli. StbA binds a cis-acting sequence, the stbDRs.
The StbA-stbDRs complex may be used to pair plasmid the host chromosome, using indirectly the bacterial partitioning system.
* StbB protein has a Walker-type ATPase motif, it favors for conjugation but is not required for plasmid stability over generations.
* StbC is an orphan protein of unknown function. StbC doesn't seem to be implicated in either partitioning or conjugation.
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.