Mobile genetic elements and their bacterial hosts are in a never-ending evolutionary dance, giving rise to a multitude of defense and evasion systems for protection and infection. Most of these defense systems are still uncharacterized or poorly understood, and a substantial number of them have conserved domains of unknown function, with the potential to uncover completely new molecular mechanisms. Understanding the molecular mechanisms of these defense systems could lead to the development of unique new biotechnological tools, as has been the case for other bacterial defense systems such as CRISPR-Cas and restriction/modification systems.

The objective of this project is to characterize the molecular mechanism by which Druantia type III (DTIII) defense system protects bacteria against bacteriophage infection. This system consists of a two-gene operon, druE and druH. The druE gene, characteristic of the Druantia family, encodes a large putative protein composed of ATP-dependent SF2 helicase domains, whereas the druH gene has no known protein domains. So far, we characterized a set of phages that are sensitive to DTIII defense system, both by expression in a heterologous strain and by deletion/complementation of the defense system inside the native strain. Most of those phages are known to be sensitive to restriction modification systems, and present only few epigenetic modifications. Moreover, mutations in the predicted helicase domains of DruE lead to a loss of the defense activity. Using recombinant DruE, we could show that the latest interacts with ssDNA and presents an uncommon mode of nuclease activity depending on the size of its substrate. Those results suggest a defense mechanism based on recognition and cleavage of the invading phage DNA expanding our understanding of the many recently discovered defenses encoded by bacteria.

References

1.Doron, S. et al. Systematic discovery of antiphage defense systems in the microbial pangenome. Science 359, eaar4120 (2018).