A Type Ib ParB protein involved in plasmid partitioning in a gram-positive bacterium

Our current understanding of segregation of prokaryotic plasmids has been derived mainly from the study of the gram-negative bacterial plasmids. We previously reported a replicon of the cryptic plasmid from a gram-positive bacterium, Leifsonia xyli subsp. cynodontis. The replicon contains a putative plasmid partition cassette including a Walker-type ATPase followed by open reading frame 4 without sequence homologue. Here we reported that the orf4 gene was essential for maintaining the plasmid stability in L. xyli subsp. cynodontis. Furthermore, the purified orf4 protein specifically and cooperatively bound to direct repeat sequences located upstream of the parA gene in vitro, indicating that orf4 is a parB gene and that the direct repeat DNA sequences constitute a partition site, parS. The location of parS and the features of ParA and ParB proteins suggest that this plasmid partition cassette belongs to type Ib, representing the first type Ib cassette identified from a gram-positive bacterial plasmid.