User:Maonao/sandbox/pnd

=PndA-PndB toxin-antitoxin system= The PndA-PndB toxin-antitoxin system of the R483 plasmid is homologous to the hok/sok system of R1. Like the hok/sok system, it performs a post-segregational killing function, ensuring that all surviving daughter cells inherit the R483 plasmid. The system consists of pndA mRNA which is relatively stable and codes for the toxic protein PndA, pndC mRNA, a regulatory element and pndA mRNA, an antitoxin with complementarity to pndC.

Mechanism of regulation and toxicity
The suspected method of regulation has not been directly tested for this system, however due to the strong similarity between this system and the hok/sok system, a mechanism of regulation has been inferred based on the structure of the mRNA and the mechanism used to regulate hok expression. pndC almost completely overlaps pndA and is partly overlapped by pndB. It appears that translation of pndA'  is dependent on translation of pndC, so regulation of expression of the PndA toxin can be achieved indirectly by regulating translation of pndC. It is thought that the initial mRNA transcript of pndA'  is translationally inactive, and in plasmid-containing cells it will slowly bind pndB mRNA and be cleaved, or it will be specifically cleaved on the 3' end and rapidly bind pndB, which indirectly inhibits translation of pndA'  by regulating translation of pndC and causing cleavage. In a plasmid free cell, pndB degrades rapidly and remaining pndA mRNA is processed into translationally active 3'-truncated mRNA which yields the PndA toxic protein, killing the cell. The mechanism by which PndA causes toxicity is not known, however similarity between the PndA toxin sequence and the hok toxin sequence suggests that they make have similar functions.