The cell pole: the site of cross talk between the DNA uptake and genetic recombination machinery

Abstract

Natural transformation is a programmed mechanism characterized by binding of free double-stranded (ds) DNA from the environment to the cell pole in rod-shaped bacteria. In Bacillus subtilis some competence proteins, which process the dsDNA and translocate single-stranded (ss) DNA into the cytosol, recruit a set of recombination proteins mainly to one of the cell poles. A subset of single-stranded binding proteins, working as “guardians”, protect ssDNA from degradation and limit the RecA recombinase loading. Then, the “mediators” overcome the inhibitory role of guardians, and recruit RecA onto ssDNA. A RecA·ssDNA filament searches for homology on the chromosome and, in a process that is controlled by “modulators”, catalyzes strand invasion with the generation of a displacement loop (D-loop). A D-loop resolvase or “resolver” cleaves this intermediate, limited DNA replication restores missing information and a DNA ligase seals the DNA ends. However, if any step fails, the “rescuers” will repair the broken end to rescue chromosomal transformation. If the ssDNA does not share homology with resident DNA, but it contains information for autonomous replication, guardian and mediator proteins catalyze plasmid establishment after inhibition of RecA. DNA replication and ligation reconstitute the molecule (plasmid transformation). In this review, the interacting network that leads to a cross talk between proteins of the uptake and genetic recombination machinery will be placed into prospective.

Bacillus subtilis DprA Recruits RecA onto Single-stranded DNA and Mediates Annealing of Complementary Strands Coated by SsbB and SsbA

“…In Bacillus subtilis, only a small fraction of cells differentiate and become naturally competent (1). These genetically programmed cells have distinct physiological characteristics that include the transient inability to synthesize DNA of its single genome or undergo cell division (1,2). A common core of at least 20 pole-localized proteins, which are either associated with the membrane or cytosolic, are temporarily expressed and collectively dedicated to the internalization, processing, and recombination of the incoming ssDNA with the resident chromosome or with itself to promote plasmid establishment (1,2).…”

“…With few exceptions, natural transformation, which is a widely distributed mechanism for genetic recombination (GR) 4 in many bacterial genera, is transiently induced (competence state) (1,3). The competence machinery actively processes exogenous dsDNA and takes up the internalized ssDNA to replace homologous (or partially homologous) chromosomal sequences in a mechanism catalyzed by RecA with the help of accessory factors (2,3). The GR mechanism, which entails unidirectional recombination of genes between different lineages, represents a form of bacterial sex that contributes to diversity, adaptation, and also the emergence of pathogens and antibiotic resistance variants (2).…”

“…segments among bacteria (1,2). With few exceptions, natural transformation, which is a widely distributed mechanism for genetic recombination (GR) 4 in many bacterial genera, is transiently induced (competence state) (1,3).…”

“…These genetically programmed cells have distinct physiological characteristics that include the transient inability to synthesize DNA of its single genome or undergo cell division (1,2). A common core of at least 20 pole-localized proteins, which are either associated with the membrane or cytosolic, are temporarily expressed and collectively dedicated to the internalization, processing, and recombination of the incoming ssDNA with the resident chromosome or with itself to promote plasmid establishment (1,2). The membrane-associated proteins, which form the DNA uptake machinery, bind environmental dsDNA, degrade one of the strands to produce ssDNA, and internalize it into the cytosol (1).…”

“…The competence machinery actively processes exogenous dsDNA and takes up the internalized ssDNA to replace homologous (or partially homologous) chromosomal sequences in a mechanism catalyzed by RecA with the help of accessory factors (2,3). The GR mechanism, which entails unidirectional recombination of genes between different lineages, represents a form of bacterial sex that contributes to diversity, adaptation, and also the emergence of pathogens and antibiotic resistance variants (2). In Bacillus subtilis, only a small fraction of cells differentiate and become naturally competent (1).…”