Transcriptional Interference by a Complex Formed at the Centromere-Like Partition Site of Plasmid P1

Abstract

The partition site, parS, promotes accurate segregation of the replicated P1 plasmid to daughter cells when the P1-encoded ParA and ParB proteins are supplied. The parS site was inserted into the Escherichia coli chromosome between the promoter and the structural gene for ␤-galactosidase, lacZ. There was little interference with lacZ expression when ParA and ParB were supplied in trans. However, when a mutant ParA protein, ParAM314I, was supplied along with ParB, expression of lacZ was shut down. ParAM314I, ParB, and parS appear to form a nucleoprotein complex that blocks transcription. Mutations in parA and parB that relieved the parAM314I-dependent block were found. In addition, new mutations which impose the block were selected. Five of the latter mapped to parA and one to parB; all had a propagation-defective phenotype (Par PD ) similar to that of parAM314I. Thus, whereas a null par mutant P1 plasmid segregates its DNA randomly, these mutants prevent even random distribution of the plasmid. We propose that ParA protein normally interacts transiently with the ParB-parS complex for partition to proceed but that the mutations block ParA dissociation. This "permanent" ParA-ParB-parS complex acts as a transcription block. Consistent with this hypothesis, we found that three of the seven blocking mutations lie within regions of ParA and ParB that are known to interact with each other. When the transcription block is imposed, regional silencing of nearby genes occurs. However, the requirement for ParA and a mutant parA or parB allele distinguishes the transcription block from the regional ParB-dependent gene silencing previously described.

Borrelia burgdorferi cp32 BpaB Modulates Expression of the Prophage NucP Nuclease and SsbP Single-Stranded DNA-Binding Protein

“…Almost all replicons encode two unique proteins that have been associated with maintenance: a ParA/SopA homolog and BpaB, leading to speculation that BpaB may function in a manner analogous to the ParB/SopB proteins of other bacterial replicons (5,11,20,30,54,55). In addition to their roles in plasmid maintenance, some ParB/ SopB proteins directly regulate the expression of plasmid genes (4,19,31,46,47). This and previous studies indicate that cp32-encoded BpaB proteins are also regulatory factors (30).…”

BpaB and EbfC DNA-Binding Proteins Regulate Production of the Lyme Disease Spirochete's Infection-Associated Erp Surface Proteins

“…6A). Such a mechanism of transcriptional repression has been proposed for other bacterial DNA-binding proteins that similarly spread along DNA (8,20,28,39,42,53,54).…”

“…BpaB plays a role in maintenance of the Erp-encoding cp32 prophages, possibly analogous to the ParB/SopB proteins of other bacterial replicons (9,21,65,66). Related to that point, several such plasmid maintenance proteins also function as transcriptional repressors through mechanisms like that proposed for BpaB and the erp operons (8,20,39,53,54). Although the spirochete phylum diverged from the rest of the kingdom Eubacteria many millions of years ago, the retention of EbfC homologs by almost all members of the Eubacteria suggests that these proteins perform central, critical functions for many prokaryotes.…”

Topoisomerase I (TopA) Is Recruited to ParB Complexes and Is Required for Proper Chromosome Organization during Streptomyces coelicolor Sporulation

“…Upon binding to parS sites as dimers, ParB is thought to polymerize along the adjacent region to form a higherorder complex, possibly a nucleoprotein filament (9,10). ParB binding is crucial for the positioning of the oriC regions within the cell and affects the positioning of other chromosomal segments (11).…”