A Halocin Promotes DNA Uptake in Haloferax mediterranei

Halocins are antimicrobial peptides or proteins that are produced by halophilic archaea. Although their function in inhibiting the growth of closely related haloarchaeal strains is well known, other physiological functions of halocins have also been proposed in recent years. To unveil the possible function and mechanism of halocins in DNA uptake, the halocin H4 producing strain Haloferax mediterranei DF50-Delta EPS (incapable of EPS production) was used in this study. We found that deletion of the halH4 resulted in the strain DF50-Delta EPSAhalH4 which exhibited loss of natural DNA uptake ability. Moreover, supernatants of the halocin producing strain were capable of inducing the ability to uptake DNA. Obviously, halocin is likely responsible for inducing DNA uptake. Cell surface ultrastructures of these strains are varied from strains DF50-Delta EPS to DF50-Delta EPSAhalH4. The cell surface of strain DF50-Delta EPS is rough due to numerous pinholes, while that of the strain DF50-Delta EPS Delta halH4 is smooth without visible pinholes. The morphology of the halH4 complemented strain, DF50-Delta EPS Delta halH4::H4, shows an intermediate phenotype between strains DF50-Delta EPS and DF50-Delta EPS Delta halH4. We speculate that halocin H4 may accelerate DNA uptake by perforating the cell surface ultrastructure. The halocin H4 may represent a novel inducer or activator of DNA uptake in Hfx. mediterranei. IMPORTANCE Halocin H4 (HalH4) secreted by Haloferax mediterranei has always been taken as a typical antimicrobial protein. In this work, we found that deletion of the halocin H4 - encoding gene, halH4, blocks the DNA uptake in Hfx. mediterranei by changing the cell surface ultrastructure. It is the first study on the function of halocins in natural environments, substantially contributing to new knowledge in this domain.