Abstract
Bacterial centromeres–also called parS, are cis-acting DNA sequences which, together with the proteins ParA and ParB, are involved in the segregation of chromosomes and plasmids. The specific binding of ParB to parS nucleates the assembly of a large ParB/DNA complex from which ParA—the motor protein, segregates the sister replicons. Closely related families of partition systems, called Bsr, were identified on the chromosomes and large plasmids of the multi-chromosomal bacterium Burkholderia cenocepacia and other species from the order Burkholeriales. The centromeres of the Bsr partition families are 16 bp palindromes, displaying similar base compositions, notably a central CG dinucleotide. Despite centromeres bind the cognate ParB with a narrow specificity, weak ParB-parS non cognate interactions were nevertheless detected between few Bsr partition systems of replicons not belonging to the same genome. These observations suggested that Bsr partition systems could have a common ancestry but that evolution mostly erased the possibilities of cross-reactions between them, in particular to prevent replicon incompatibility. To detect novel similarities between Bsr partition systems, we have analyzed the binding of six Bsr parS sequences and a wide collection of modified derivatives, to their cognate ParB. The study was carried out by Surface Plasmon Resonance imaging (SPRi) mulitplex analysis enabling a systematic survey of each nucleotide position within the centromere. We found that in each parS some positions could be changed while maintaining binding to ParB. Each centromere displays its own pattern of changes, but some positions are shared more or less widely. In addition from these changes we could speculate evolutionary links between these centromeres.
Highlights
Partition (Par) systems were discovered on low-copy number plasmids to whom they ensure active segregation and stability over generations
The goal of this study was to analyse the interaction of six ParB proteins originating from six Burkholderiales replicons with their cognate parS and many derivatives, in order to get insights on the specificities and similarities of these Par systems
Crude Escherichia coli extracts over-expressing one ParB were used in this study, as they are at least as sensitive as purified ParB proteins in detecting DNA probes in Surface Plasmon Resonance imaging (SPRi) assays [9]
Summary
Partition (Par) systems were discovered on low-copy number plasmids to whom they ensure active segregation and stability over generations. They consist of three elements: a NTPase, called ParA, a DNA-binding protein called ParB, and a cis-acting sequence—the target of ParB, referred to as centromere or parS site. Par B binds to parS forming a nucleo-protein complex with which ParA interacts to lead the segregation of daughter replicons [3]. While in types II and III the NTPase polymerization is involved in driving plasmids apart, the segregation mechanism remains unclear in type I, the most common Par system of plasmids and the only one found on chromosomes
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