Streptococcus pyogenes pSM19035 requires dynamic assembly of ATP-bound ParA and ParB on parS DNA during plasmid segregation.

The accurate partitioning of Firmicute plasmid pSM19035 at cell division depends on ATP binding and hydrolysis by homodimeric ATPase delta(2) (ParA) and binding of omega(2) (ParB) to its cognate parS DNA. The 1.83 A resolution crystal structure of delta(2) in a complex with non-hydrolyzable ATPgammaS reveals a unique ParA dimer assembly that permits nucleotide exchange without requiring dissociation into monomers. In vitro, delta(2) had minimal ATPase activity in the absence of omega(2) and parS DNA. However, stoichiometric amounts of omega(2) and parS DNA stimulated the delta(2) ATPase activity and mediated plasmid pairing, whereas at high (4:1) omega(2) : delta(2) ratios, stimulation of the ATPase activity was reduced and delta(2) polymerized onto DNA. Stimulation of the delta(2) ATPase activity and its polymerization on DNA required ability of omega(2) to bind parS DNA and its N-terminus. In vivo experiments showed that delta(2) alone associated with the nucleoid, and in the presence of omega(2) and parS DNA, delta(2) oscillated between the nucleoid and the cell poles and formed spiral-like structures. Our studies indicate that the molar omega(2) : delta(2) ratio regulates the polymerization properties of (delta*ATP*Mg(2+))(2) on and depolymerization from parS DNA, thereby controlling the temporal and spatial segregation of pSM19035 before cell division.

Structures of omega repressors bound to direct and inverted DNA repeats explain modulation of transcription.

Repressor omega regulates transcription of genes required for copy number control, accurate segregation and stable maintenance of inc18 plasmids hosted by Gram-positive bacteria. omega belongs to homodimeric ribbon-helix-helix (RHH2) repressors typified by a central, antiparallel beta-sheet for DNA major groove binding. Homodimeric omega2 binds cooperatively to promotors with 7 to 10 consecutive non-palindromic DNA heptad repeats (5'-(A)/(T)ATCAC(A)/(T)-3', symbolized by -->) in palindromic inverted, converging (--><--) or diverging (<---->) orientation and also, unique to omega2 and contrasting other RHH2 repressors, to non-palindromic direct (-->-->) repeats. Here we investigate with crystal structures how omega2 binds specifically to heptads in minimal operators with (-->-->) and (--><--) repeats. Since the pseudo-2-fold axis relating the monomers in omega(2) passes the central C-G base pair of each heptad with approximately 0.3 A downstream offset, the separation between the pseudo-2-fold axes is exactly 7 bp in (-->-->), approximately 0.6 A shorter in (--><--) but would be approximately 0.6 A longer in (<---->). These variations grade interactions between adjacent omega2 and explain modulations in cooperative binding affinity of omega2 to operators with different heptad orientations.