Bacterial conjugation, also referred to as bacterial sex, is a major horizontal gene transfer mechanism through which DNA is transferred from a donor to a recipient bacterium by direct contact. Conjugation is universally conserved among bacteria and occurs in a wide range of environments (soil, plant surfaces, water, sewage, biofilms, and host-associated bacterial communities). Within these habitats, conjugation drives the rapid evolution and adaptation of bacterial strains by mediating the propagation of various metabolic properties, including symbiotic lifestyle, virulence, biofilm formation, resistance to heavy metals, and, most importantly, resistance to antibiotics. These properties make conjugation a fundamentally important process, and it is thus the focus of extensive study. Here, we review the key steps of plasmid transfer by conjugation in Gram-negative bacteria, by following the life cycle of the F factor during its transfer from the donor to the recipient cell. We also discuss our current knowledge of the extent and impact of conjugation within an environmentally and clinically relevant bacterial habitat, bacterial biofilms.

Significance: Transcription is one of the most fundamental processes for life. In eukaryotic cells, transcriptional activity is regulated to a large degree by chromosome packaging. In bacteria, despite the absence of a nuclear envelope and many of the DNA-packaging proteins of eukaryotes, the chromosome is still highly condensed into a structured object, the nucleoid. The spatial organization of transcription within the nucleoid and the effect of transcription on DNA organization remain poorly understood. In this work, we characterize how RNA polymerase accesses transcription sites on DNA, and show that active transcription can cause spatial reorganization of the nucleoid, with movement of gene loci out of the bulk of DNA as levels of transcription increase.

A race against time: Clinically relevant antimicrobial resistance is largely spread via plasmids that disperse among bacteria during conjugation. How quickly can a resistance gene be expressed after transfer? In susceptible bacterial cells, tetracycline should inhibit protein synthesis, including from the plasmid-transferred resistance gene tetA . Unexpectedly, Nolivos et al. found that TetA can be expressed despite the presence of tetracycline (see the Perspective by Povolo and Ackermann). Immediately after plasmid transfer into a cell, TetA synthesis starts because its repressor is slow to be expressed. In addition, the ubiquitous xenobiotic efflux pump AcrAB-TolC buys time for TetA translation by keeping tetracycline concentration below toxic levels. Science , this issue p. 778 ; see also p. 737

A new mechanism is identified for correct placement of the division machinery in Streptococcus pneumoniae that relies on the novel factor MapZ to form ring structures at the cell equator; these structures move apart as the cell elongates, acting as permanent markers of division sites. In many bacteria, the correct mid-cell placement of the cell division machinery —including the ring formed by the conserved tubulin homologue FtsZ — is controlled by the Min system, which prevents FtsZ polymerization at the cell poles. This system is absent in the pathogenic bacterium Streptococcus pneumoniae, however. Here Christophe Grangeasse and colleagues identify a placement mechanism that does operate in S. pneumonia. It depends on mid-cell anchored protein Z (MapZ) to form ring structures at the cell equator that separate as the cell elongates, acting as a permanent beacon for the division sites. MapZ interacts with FtsZ to position the Z-ring for division. In every living organism, cell division requires accurate identification of the division site and placement of the division machinery. In bacteria, this process is traditionally considered to begin with the polymerization of the highly conserved tubulin-like protein FtsZ into a ring that locates precisely at mid-cell1. Over the past decades, several systems have been reported to regulate the spatiotemporal assembly and placement of the FtsZ ring2,3,4,5. However, the human pathogen Streptococcus pneumoniae, in common with many other organisms, is devoid of these canonical systems and the mechanisms of positioning the division machinery remain unknown4,6. Here we characterize a novel factor that locates at the division site before FtsZ and guides septum positioning in pneumococcus. Mid-cell-anchored protein Z (MapZ) forms ring structures at the cell equator and moves apart as the cell elongates, therefore behaving as a permanent beacon of division sites. MapZ then positions the FtsZ ring through direct protein–protein interactions. MapZ-mediated control differs from previously described systems mostly on the basis of negative regulation of FtsZ assembly. Furthermore, MapZ is an endogenous target of the Ser/Thr kinase StkP, which was recently shown to have a central role in cytokinesis and morphogenesis of S. pneumoniae7,8,9. We show that both phosphorylated and non-phosphorylated forms of MapZ are required for proper Z-ring formation and dynamics. Altogether, this work uncovers a new mechanism for bacterial cell division that is regulated by phosphorylation and illustrates that nature has evolved a diversity of cell division mechanisms adapted to the different bacterial clades.

RecA bundles are shown to be important for the pairing of homologous loci that have segregated to opposite ends of the cell during DNA double-strand break repair in vivo in Escherichia coli. Although bacterial RecA protein functions as a filament during DNA strand exchange, early studies of RecA also noted that, in vivo, it formed bundles. These bundles were inactive in vitro, so were thought to be a way of storing RecA until it was needed. David Sherratt and colleagues now show that RecA bundles do have an important function in vivo. Super-resolution microscopy imaging shows that bundles are excluded from the bulk of the nucleoid and locate to the cell periphery where they facilitate the pairing of homologous loci that have segregated to opposite ends of the cell. After sister locus pairing, RecA bundles disassemble. DNA double-strand break (DSB) repair by homologous recombination has evolved to maintain genetic integrity in all organisms1. Although many reactions that occur during homologous recombination are known1,2,3, it is unclear where, when and how they occur in cells. Here, by using conventional and super-resolution microscopy, we describe the progression of DSB repair in live Escherichia coli. Specifically, we investigate whether homologous recombination can occur efficiently between distant sister loci that have segregated to opposite halves of an E. coli cell. We show that a site-specific DSB in one sister can be repaired efficiently using distant sister homology. After RecBCD processing of the DSB, RecA is recruited to the cut locus, where it nucleates into a bundle that contains many more RecA molecules than can associate with the two single-stranded DNA regions that form at the DSB. Mature bundles extend along the long axis of the cell, in the space between the bulk nucleoid and the inner membrane. Bundle formation is followed by pairing, in which the two ends of the cut locus relocate at the periphery of the nucleoid and together move rapidly towards the homology of the uncut sister. After sister locus pairing, RecA bundles disassemble and proteins that act late in homologous recombination are recruited to give viable recombinants 1–2-generation-time equivalents after formation of the initial DSB. Mutated RecA proteins that do not form bundles are defective in sister pairing and in DSB-induced repair. This work reveals an unanticipated role of RecA bundles in channelling the movement of the DNA DSB ends, thereby facilitating the long-range homology search that occurs before the strand invasion and transfer reactions.

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