Genetic engineering of marine cyanophages reveals integration but not lysogeny in T7-like cyanophages

Marine cyanobacteria of the genera Synechococcus and Prochlorococcus are the most abundant photosynthetic organisms on earth, spanning vast regions of the oceans and contributing significantly to global primary production. Their viruses (cyanophages) greatly influence cyanobacterial ecology and evolution. Although many cyanophage genomes have been sequenced, insight into the functional role of cyanophage genes is limited by the lack of a cyanophage genetic engineering system. Here, we describe a simple, generalizable method for genetic engineering of cyanophages from multiple families, that we named REEP for REcombination, Enrichment and PCR screening. This method enables direct investigation of key cyanophage genes, and its simplicity makes it adaptable to other ecologically relevant host-virus systems. T7-like cyanophages often carry integrase genes and attachment sites, yet exhibit lytic infection dynamics. Here, using REEP, we investigated their ability to integrate and maintain a lysogenic life cycle. We found that these cyanophages integrate into the host genome and that the integrase and attachment site are required for integration. However, stable lysogens did not form. The frequency of integration was found to be low in both lab cultures and the oceans. These findings suggest that T7-like cyanophage integration is transient and is not part of a classical lysogenic cycle

Etude du pilus de transformation chez streptococcus pneumoniae

Natural transformation is the ability of bacteria to actively take up and recombine extracellular DNA. This crucial process increases genome plasticity and adaptability of Gram-negative and Gram-positive bacteria through intra- and inter-species genetic exchange. S. pneumoniae is a major human pathogen responsible for severe diseases such as pneumonia, meningitis and septicemia. In this species, transformation has been linked to capsular serotype switching and reduced vaccine efficiency. Most transformable Gram-positive bacteria carry a comG operon that resembles operons encoding a widespread family of pili in Gram-negative bacteria, the type IV pili. It has been commonly proposed that the comG operon is responsible for the formation of a short pseudo-pilus filament. However, such an appendage had never been visualized in any bacterium. By mutagenesis, biochemical characterization, optical and electron microscopy techniques we were able to identify long, micrometer-sized appendages protruding from the surface of competent S. pneumoniae. We confirmed the Type IV pili nature of these appendages, we showed that they bind DNA, and are absolutely required for DNA uptake. We consequently overthrew the pseudopilus hypothesis at least in S. pneumoniae, and provided crucial information concerning the initial step of DNA uptake. We propose a revised model in which the transformation pilus acts as a “DNA trap” capturing DNA at the surface of competent cells, guiding it to the translocation channel.La transformation naturelle est la capacité de certaines bactéries à incorporer et à recombiner activement de l’ADN extra-cellulaire. Ce procédé majeur augmente la plasticité et l’adaptabilité des bactéries à Gram positif et négatif en réalisant des échanges génétiques intra- et inter-espèces. S. pneumoniae est un pathogène majeur de l’Homme. Cette bactérie est responsable d’infections sévères telles que des pneumonies, des méningites et des septicémies. Dans cette espèce, la transformation naturelle est corrélée au phénomène de changement de capsule et à la baisse d’efficacité des vaccins. La plupart des bactéries à Gram positif naturellement transformables possèdent un opéron comG, semblable aux opérons codant pour la famille des pili de type IV, extrêmement répandus chez les bactéries à Gram négatif. Il a été proposé que l’opéron comG est responsable de la formation d’un petit filament, nommé pseudo- pilus. Cependant, un tel filament n’a jamais été observé. Par des techniques de mutagenèse, de caractérisation biochimique, de microscopie optique et électronique, nous sommes parvenus à identifier des filaments de plusieurs micromètres de long à la surface de bactéries S. pneumoniae compétentes. Nous avons confirmé l’appartenance de ces filaments à la famille des pili de type IV. Par conséquent, nous avons infirmé l’hypothèse de la formation d’un pseudo-pilus par l’opérons comG chez S. pneumoniae. De plus, nous avons montré que les pili se lient à l’ADN et qu’ils sont requis pour la capture de l’ADN extra-cellulaire. Ces résultats apportent des informations cruciales concernant les premières étapes de capture de l’ADN durant la transformation naturelle. Nous proposons un nouveau modèle dans lequel le pilus agirait comme un « piège à ADN », capturant l’ADN à la surface des bactéries compétentes pour le guider jusqu’au pore d’entrée dans la cellule

A Type IV Pilus Mediates DNA Binding during Natural Transformation in Streptococcus pneumoniae

International audienceNatural genetic transformation is widely distributed in bacteria and generally occurs during a genetically programmed differentiated state called competence. This process promotes genome plasticity and adaptability in Gram-negative and Gram-positive bacteria. Transformation requires the binding and internalization of exogenous DNA, the mechanisms of which are unclear. Here, we report the discovery of a transformation pilus at the surface of competent Streptococcus pneumoniae cells. This Type IV-like pilus, which is primarily composed of the ComGC pilin, is required for transformation. We provide evidence that it directly binds DNA and propose that the transformation pilus is the primary DNA receptor on the bacterial cell during transformation in S. pneumoniae. Being a central component of the transformation apparatus, the transformation pilus enables S. pneumoniae, a major Gram-positive human pathogen, to acquire resistance to antibiotics and to escape vaccines through the binding and incorporation of new genetic material