The P1 plasmid prophage is faithfully partitioned by a high affinity nucleo
protein complex assembled at the centromere-like parS site. This partition
complex is composed of P1 ParB and Escherichia coli integration host factor
(IHF), bound specifically to parS. We have investigated the assembly of Pa
rB at parS and its stoichiometry of binding. Measured by gel mobility shift
assays, ParB and IHF bind tightly to parS and form a specific complex, cal
led I + B1. We observed that as ParB concentration was increased, a second,
larger complex (I + B2) formed, followed by the formation of larger comple
xes, indicating that additional ParB molecules joined the initial complex.
Shift Western blotting experiments indicated that the I + B2 complex contai
ned twice as much ParB as the I + B1 complex. Using mixtures of ParB and a
larger polyhistidine-tagged version of ParB (His-ParB) in DNA binding assay
s, we determined that the initial I + B1 complex contains one dimer of ParB
. Therefore, one dimer of ParB binds to its recognition sequences that span
an IHF-directed bend in parS. Once this complex forms, a second dimer can
join the complex, but this assembly requires much higher ParB concentration
s.