title: P1 plasmid segregation: accurate redistribution by dynamic plasmid pairing and separation.
abstract: Low-copy-number plasmids, such as P1 and F, encode a type Ia partition system (P1par or Fsop) for active segregation of copies to daughter cells. Typical descriptions show a single central plasmid focus dividing and the products moving to the cell quarter regions, ensuring segregation. However, using improved optical and analytical tools and large cell populations, we show that P1 plasmid foci are very broadly distributed. Moreover, under most growth conditions, more than two foci are frequently present. Each focus contains either one or two plasmid copies. Replication and focus splitting occur at almost any position in the cell. The products then move rapidly apart for approximately 40% of the cell length. They then tend to maintain their relative positions. The segregating foci often pass close to or come to rest close to other foci in the cell. Foci frequently appear to fuse during these encounters. Such events occur several times in each cell and cell generation on average. We argue that foci pair with their neighbors and then actively separate again. The net result is an approximately even distribution of foci along the long cell axis on average. We show mathematically that trans-pairing and active separation could greatly increase the accuracy of segregation and would produce the distributions of foci that we observe. Plasmid pairing and separation may constitute a novel fine-tuning mechanism that takes the basic pattern created when plasmids separate after replication and converts it to a roughly even pattern that greatly improves the fidelity of plasmid segregation.
authors.affiliation: Gene Regulation and Chromosome Biology Laboratory, National Cancer Institute, CCR, NCI-Frederick, Frederick, MD 21702-1201, USA.
authors.forename: Manjistha
authors.initials: M
authors.lastname: Sengupta
authors.forename: Henrik Jorck
authors.initials: HJ
authors.lastname: Nielsen
authors.forename: Brenda
authors.initials: B
authors.lastname: Youngren
authors.forename: Stuart
authors.initials: S
authors.lastname: Austin
mesh_terms: D017101:Bacteriophage P1 / Q000235:genetics* / Q000254:growth & development*; D002455:Cell Division; D004261:DNA Replication; D004279:DNA, Viral / Q000235:genetics* / Q000378:metabolism*; D004926:Escherichia coli / Q000821:virology*; D008954:Models, Biological; D010957:Plasmids / Q000378:metabolism*
chemical_list: D004279:DNA, Viral