Abstract
Bacterial extracellular nucleases have multiple functions in processes as diverse as nutrient acquisition, natural transformation, biofilm formation, or defense against neutrophil extracellular traps (NETs). Here we explored the properties of ExeM in Shewanella oneidensis MR-1, an extracellular nuclease, which is widely conserved among species of Shewanella, Vibrio, Aeromonas, and others. In S. oneidensis, ExeM is crucial for normal biofilm formation. In vitro activity measurements on heterologously produced ExeM revealed that this enzyme is a sugar-unspecific endonuclease, which requires Ca2+ and Mg2+/Mn2+ as co-factors for full activity. ExeM was almost exclusively localized to the cytoplasmic membrane fraction, even when a putative C-terminal membrane anchor was deleted. In contrast, ExeM was not detected in medium supernatants. Based on the results we hypothesize that ExeM predominantly interacts with DNA in close proximity to the cell, e.g., to promote biofilm formation and defense against NETs, or to control uptake of DNA.
Highlights
Extracellular DNA ubiquitously occurs in terrestrial, marine and fresh-water habitats as the product of passive or active cell lysis or active DNA transport (Vlassov et al, 2007; Ibánez de Aldecoa et al, 2017)
We assigned the primary function of ExeM in S. oneidensis MR-1 to biofilm formation (Gödeke et al, 2011a; Heun et al, 2012)
As the nuclease ExeM is conserved in Shewanellaceae, Vibrionaceae, Aeromonadaceae, and Pseudoalteromonadacae, we expect our findings to, extent to other bacterial species
Summary
Extracellular DNA ubiquitously occurs in terrestrial, marine and fresh-water habitats as the product of passive or active cell lysis or active DNA transport (Vlassov et al, 2007; Ibánez de Aldecoa et al, 2017). To test the in vivo activity of ExeM and corresponding mutant versions, the appropriate S. oneidensis strains were incubated on DNA agar and the extracellular nucleolytic activity was determined as a measure of clearance of turbidity around the colonies due to DNA degradation (Figure 3C).
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