The Interplay between Different Stability Systems Contributes to Faithful Segregation:Streptococcus pyogenespSM19035 as a Model

Abstract

The Streptococcus pyogenes pSM19035 low-copy-number θ-replicating plasmid encodes five segregation (seg) loci that contribute to plasmid maintenance. These loci map outside of the minimal replicon. The segA locus comprises β2 recombinase and two six sites, and segC includes segA and also the γ topoisomerase and two ssiA sites. Recombinase β2 plays a role both in maximizing random segregation by resolving plasmid dimers (segA) and in catalyzing inversion between two inversely oriented six sites. segA, in concert with segC, facilitates replication fork pausing at ssiA sites and overcomes the accumulation of "toxic" replication intermediates. The segB1 locus encodes ω, ε, and ζ genes. The short-lived ε2 antitoxin and the long-lived ζ toxin form an inactive ζε2ζ complex. Free ζ toxin halts cell proliferation upon decay of the ε2 antitoxin and enhances survival. If ε2 expression is not recovered, by loss of the plasmid, the toxin raises lethality. The segB2 locus comprises δ and ω genes and six parS sites. Proteins δ2 and ω2, by forming complexes with parS and chromosomal DNA, pair the plasmid copies at the nucleoid, leading to the formation of a dynamic δ2 gradient that separates the plasmids to ensure roughly equal distribution to daughter cells at cell division. The segD locus, which comprises ω2 (or ω2 plus ω22) and parS sites, coordinates expression of genes that control copy number, better-than-random segregation, faithful partition, and antibiotic resistance. The interplay of the seg loci and with the rep locus facilitates almost absolute plasmid stability.

The interaction of ω2with the RNA polymerase β’ subunit functions as an activation to repression switch

“…Resistance to glycopeptides, macrolides, pleuromutilins, phenicols, linezolid and other antibiotics among Gram-positive cocci is generally linked to plasmids whose copy number control, partitioning and post-segregational killing is regulated by the ω cassette ( 1 , 2 ). This cassette contains sequences that encode the ω or ω2 gene products ( 1 ).…”

“…Resistance to glycopeptides, macrolides, pleuromutilins, phenicols, linezolid and other antibiotics among Gram-positive cocci is generally linked to plasmids whose copy number control, partitioning and post-segregational killing is regulated by the ω cassette ( 1 , 2 ). This cassette contains sequences that encode the ω or ω2 gene products ( 1 ). It is thus important to understand how homodimeric ω (ω 2 ) (or ω2 [ω2 2 ]) functions, not only because of its intrinsic biological interest, but also because of its relevance to antibiotic resistance transmission ( 1 , 2 ).…”

“…This cassette contains sequences that encode the ω or ω2 gene products ( 1 ). It is thus important to understand how homodimeric ω (ω 2 ) (or ω2 [ω2 2 ]) functions, not only because of its intrinsic biological interest, but also because of its relevance to antibiotic resistance transmission ( 1 , 2 ). Plasmids of the inc 18 family are commonly found in Enterococcus and Streptococcus .…”

“…pSM19035, pIP501 and pAMβ1) revealed that ω 2 controlled the expression of the copy control gene copS . Also mediated by ω 2 was the expression of plasmid partition genes, such as δ and ω, and toxin-antitoxin systems, such as the ωϵζ operon (Figure 1A ) ( 1 ). The inc 18 plasmids persist in the population through a variety of mechanisms controlled by ω 2 .…”

“…The ω 2 protein is a ParB homologue and binds to parS centromers. In concert with δ 2 (a ParA ATPase), ω 2 is involved in accurate plasmid partitioning and coupling plasmid replication to faithful segregation ( 1 , 3 – 5 ). All three parS sites (Figure 1A ) can cause partition-mediated incompatibility ( 5 ).…”