Active segregation by the Bacillus subtilis partitioning system in Escherichia coli

Abstract

Bacterial genes required for proper partitioning consist of two transacting genes that encode proteins and a cis-acting gene that functions like a centromere. Plasmids actively partitioning by means of these genes migrate from midcell to the cell quarters and are tethered to these sites until the cells divide. Previously the partitioning genes were mainly found on plasmids and phages in Escherichia coli. However, progress in genome sequencing reveals that partitioning genes are ubiquitous in many bacterial plasmids and chromosomes. Each homologue of the two transacting genes belongs to a family, ParA or ParB. Moreover, phylogenic analysis of members of the ParA and ParB families indicates that each member falls into a chromosomal group or an extrachromosomal group. It is known that the parAB genes in the chromosomal group are located on relatively conserved chromosomal regions in several bacterial species. This suggests that the parAB genes were transferred from a chromosome to plasmids and phages, so the genes have diverged among bacterial species. To support this possibility, we show that the Bacillus subtilis Soj and Spo0J members of the ParAB families are responsible for the specific localization of plasmids at cell quarters in E. coli and can function as partition proteins. Host factors to tether actively partitioning plasmids at subcellular sites may be conserved in Gram-negative and Grampositive bacteria so that phages and plasmids with the ParAB partitioning system can be stably inherited in host cells across bacterial species. L ow copy-number plasmids are stably maintained in host Escherichia coli cells because of active partitioning mechanisms; in the cases of F plasmid and P1 phage, which is a plasmid in lysogenized host cells, these are specified by the sopABC and parABS genes, respectively (1). SopA and P1͞ParA are similar in amino acid sequence and function, as are SopB and P1͞ParB. Both sopC and parS are cis-acting sites that function as centromere-like elements. In the case of the sopABC mechanism, the SopB protein binds to sopC. SopA interacts with the SopB-sopC complex to ensure proper segregation. Plasmids actively partitioned by the sopABC or parABS systems are localized at specific subcellular sites throughout the cell-division cycle (2-4). In newborn cells, the plasmids are localized at midcell. Replicated plasmids are then rapidly relocalized at the 1͞4 and 3͞4 positions of cell length. The SopAB-sopC complex probably binds to unidentified host factors that localize at the cell quarters, tethering actively partitioning plasmids at these specific subcellular sites.Many homologues of SopAB and P1͞ParAB have been identified in various plasmids and phages, and in fact several members are involved in the accurate partitioning of extrachromosomal elements (1, 5-7). We therefore call these homologues the ParA and ParB families. Recent genome searches have revealed that genes of members of both families are found in many bacterial genomes (8, 9). Homologues of the parAB genes are often located in...

Towards understanding the molecular basis of bacterial DNA segregation

“…Research has shown that 8 of the 10 sequences are bound by Spo0J in vivo, and that the presence of one such site on an otherwise unstable plasmid stabilizes the plasmid in a Soj-and Spo0J-dependent manner, demonstrating that the site, parS, can function as a partitioning site (Lin & Grossman 1998). Moreover, it has since been shown that both the B. subtilis and P. aeruginosa partitioning machineries can function in E. coli (Godfrin-Estevenon et al 2002;Yamaichi & Niki 2000).…”

“…Importantly, the chromosomal partitioning proteins can be engineered to function in the same way as plasmid par proteins to stabilize an otherwise unstable replicon and direct its correct positioning within the cell (Godfrin-Estevenon et al 2002;Lin & Grossman 1998;Yamaichi & Niki 2000). Furthermore, null mutants of ParB are lethal in Caulobacter crescentus (Mohl et al 2001), and mutations in spo0J in B. subtilis result in the formation of up to 3% anucleate cells (Ireton et al 1994).…”

“…However, elucidation of that role has been complicated by the lack of a strong missegregation phenotype in deletion mutants. Chromosomal homologues of ParA and ParB (sometimes denoted as Soj and Spo0J, because of their involvement in sporulation) have been identified in a wide range of both Gram-negative and Gram-positive bacteria (Yamaichi & Niki 2000), covering most of the bacterial phylogenetic tree. The notable exceptions are the enteric bacteria, including E. coli and the gamma subdivision of proteobacteria (Yamaichi & Niki 2000).…”

“…Chromosomal homologues of ParA and ParB (sometimes denoted as Soj and Spo0J, because of their involvement in sporulation) have been identified in a wide range of both Gram-negative and Gram-positive bacteria (Yamaichi & Niki 2000), covering most of the bacterial phylogenetic tree. The notable exceptions are the enteric bacteria, including E. coli and the gamma subdivision of proteobacteria (Yamaichi & Niki 2000). Although Soj/Spo0J were first discovered in B. subtilis, because of their role in chromosome segregation during sporulation, many chromosomal homologues of ParA and ParB are found in organisms that are not known to sporulate, suggesting a more fundamental role for these proteins.…”

“…However, Soj is required for the stabilization of parS-containing plasmids (Yamaichi & Niki 2000). Soj and other ParA proteins are members of a large family of ATPases that include the bacterial cell division regulator MinD, nitrogenase iron protein involved in biological nitrogen fixation and the anion pump, ATPase ArsA (Koonin 1993).…”