Fine-tuning cellular levels of DprA ensures transformant fitness in the human pathogen Streptococcus pneumoniae
Natural genetic transformation is a widespread mechanism of bacterial horizontal gene transfer. Transformation involves the internalization of exogenous DNA as single strands, followed by chromosomal integration via homologous recombination, promoting acquisition of new genetic traits. Transformation occurs during a distinct physiological state called competence, during which all proteins required to transform are produced. In the human pathogen Streptococcus pneumoniae, competence is controlled by a two-component system ComDE, which is induced by an exported peptide pheromone. DprA is universal among transformable species, strongly and specifically induced during pneumococcal competence, and crucial for pneumococcal transformation. Pneumococcal DprA plays three crucial roles in transformation and competence. Firstly, DprA protects internalized single-stranded (ss) DNA from degradation. Secondly, DprA loads the homologous recombinase RecA onto transforming ssDNA to promote transformation. Finally, DprA interacts with the response regulator ComE to shut-off pneumococcal competence. Pneumococcal shut-off has been linked to physiology, with long growth delays in competent dprA- cells. Here, we explored the effect of altering the cellular levels of DprA on these three roles. High cellular levels of DprA were not required for the primary role of DprA as a transformation-dedicated recombinase loader or for protection of transforming ssDNA. In contrast, full expression of dprA was required for optimal competence shut-off. Full expression of dprA was also crucial for transformant fitness. High cellular levels of DprA in competent cells thus ensure the fitness of pneumococcal transformants by promoting competence shut-off. This promotes survival and propagation of transformants, thus maximizing the adaptive potential of this human pathogen.\n\nImportanceTransformation is a widespread mechanism of horizontal gene transfer that allows bacteria to acquire new genetic traits by internalizing foreign DNA and integrating it into their chromosomes. Transformation occurs during a transient physiological state called competence. DprA is conserved in transformable species and crucial for the protection and integration of transforming DNA. In the human pathogen Streptococcus pneumoniae, DprA is highly abundant and is also crucial for competence shut-off. Here, we show that high DprA expression is not required for transformation. In contrast, full expression of dprA was required for competence shut-off and transformant fitness. These findings thus link high cellular levels of DprA to survival and propagation of pneumococcal transformants, maximizing the adaptive potential of this human pathogen.