pSM19035 is a low-copy-number theta-replicating plasmid, which belongs to the 
Inc18 family. Plasmids of this family, which show a modular organization, are 
functional in evolutionarily diverse bacterial species of the Firmicutes Phylum. 
This review summarizes our understanding, accumulated during the last 20 years, 
on the genetics, biochemistry, cytology and physiology of the five pSM19035 
segregation (seg) loci, which map outside of the minimal replicon. The segA 
locus plays a role both in maximizing plasmid random segregation, and in 
avoiding replication fork collapses in those plasmids with long inverted 
repeated regions. The segB1 locus, which acts as the ultimate determinant of 
plasmid maintenance, encodes a short-lived epsilon(2) antitoxin protein and a 
long-lived zeta toxin protein, which form a complex that neutralizes zeta 
toxicity. The cells that do not receive a copy of the plasmid halt their 
proliferation upon decay of the epsilon(2) antitoxin. The segB2 locus, which 
encodes two trans-acting, ParA- and ParB-like proteins and six cis-acting parS 
centromeres, actively ensures equal or roughly equal distribution of plasmid 
copies to daughter cells. The segC locus includes functions that promote the 
shift from the use of DNA polymerase I to the replicase (PolC-PolE DNA 
polymerases). The segD locus, which encodes a trans-acting transcriptional 
repressor, omega(2), and six cis-acting cognate sites, coordinates the 
expression of genes that control copy number, better-than-random segregation and 
partition, and assures the proper balance of these different functions. Working 
in concert the five different loci achieve almost absolute plasmid maintenance 
with a minimal growth penalty.