P1 ParA interacts with the P1 partition complex at parS and an ATP-ADP switch controls ParA activities

The partition system of pi plasmids is composed of two proteins, ParA and P arB, and a cis-acting site parS, parS is wrapped around ParB and Escherichi a call IHF protein in a higher order nucleoprotein complex called the parti tion complex. ParA is an ATPase that autoregulates the expression of the pa r operon and has an essential but unknown function in the partition process . In this study we demonstrate a direct interaction between ParA and the pi partition complex. The interaction was strictly dependent on ParB and ATP, The consequence of this interaction depended on the ParB concentration. At high ParB levels, ParA was recruited to the partition complex via a ParA-P arB interaction, but at low ParB levels, ParA removed or disassembled ParB from the partition complex. ADP could not support these interactions, but c ould promote the site-specific DNA binding activity of ParA to parOP, the o perator of the par operon. Conversely, ATP could not support a stable inter action of ParA with parOP in this assay, Our data suggest that ParA-ADP is the repressor of the par operon, and ParA-ATP, by interacting with the part ition complex, plays a direct role in partition. Therefore, one role of ade nine nucleotide binding and hydrolysis by ParA is that of a molecular switc h controlling entry into two separate pathways in which ParA plays differen t roles.