The Interplay between Different Stability Systems Contributes to Faithful Segregation: Streptococcus pyogenes pSM19035 as a Model

Abstract

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 β 2 recombinase and two six sites, and segC includes segA and also the γ topoisomerase and two ssiA sites. Recombinase β 2 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 ε 2 antitoxin and the long-lived ζ toxin form an inactive ζε 2 ζ complex. Free ζ toxin halts cell proliferation upon decay of the ε 2 antitoxin and enhances survival. If ε 2 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 δ 2 and ω 2 , by forming complexes with parS and chromosomal DNA, pair the plasmid copies at the nucleoid, leading to the formation of a dynamic δ 2 gradient that separates the plasmids to ensure roughly equal distribution to daughter cells at cell division. The segD locus, which comprises ω 2 (or ω 2 plus ω2 2 ) 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.