SOS-Independent Pyocin Production in P. aeruginosa Is Induced by XerC Recombinase Deficiency

Abstract

Pseudomonas aeruginosa is a versatile and ubiquitous bacterium that frequently infects humans as an opportunistic pathogen. P. aeruginosa competes with other strains within the species by producing killing complexes termed pyocins, which are only known to be induced by cells experiencing DNA damage and the subsequent SOS response. Here, we discovered that strains lacking a recombinase enzyme called XerC strongly produce pyocins independently of the SOS response.

DNA Damage-Inducible Pyocin Expression Is Independent of RecA in xerC -Deleted Pseudomonas aeruginosa

“…In both P. aeruginosa and Escherichia coli , XerC acts with a second recombinase, XerD, to catalyze site-specific recombination at chromosomal dif sites to decatenate replicated chromosomes ( 13 – 16 ). We found that genetic inactivation of XerC recombinase activity via Phe substitution for the nucleophilic Tyr residue required for DNA cleavage and subsequent recombination ( 17 ) at the active site (XerC Y272F ) increased pyocin expression, but to a substantially lesser degree than the full xerC deletion ( 12 ). This finding suggested that loss of recombinase activity contributes to but does not fully explain the elevated pyocin expression of strains deleted for xerC .…”

“…The subsequent lysis of pyocin-producing cells appears to be one way that fluoroquinolones kill P. aeruginosa cells, as cells mutant for recA or pyocin genes show greater resistance to such antibiotics ( 11 ). We recently discovered that strains lacking the tyrosine recombinase XerC exhibit markedly elevated basal pyocin expression that is independent of RecA ( 12 ). The overproduced pyocins released by Δ xerC strains are effective in killing sensitive strains, and xerC complementation restores wild-type levels of pyocin production ( 12 ).…”

“…We recently discovered that strains lacking the tyrosine recombinase XerC exhibit markedly elevated basal pyocin expression that is independent of RecA ( 12 ). The overproduced pyocins released by Δ xerC strains are effective in killing sensitive strains, and xerC complementation restores wild-type levels of pyocin production ( 12 ). Interestingly, we also found that treatment of Δ xerC cells with ciprofloxacin resulted in a substantial further increase in pyocin expression over their already-elevated basal levels ( 12 ).…”

“…The overproduced pyocins released by Δ xerC strains are effective in killing sensitive strains, and xerC complementation restores wild-type levels of pyocin production ( 12 ). Interestingly, we also found that treatment of Δ xerC cells with ciprofloxacin resulted in a substantial further increase in pyocin expression over their already-elevated basal levels ( 12 ). One attractive explanation for this further stimulation might be that ciprofloxacin treatment additionally activates the DNA damage-inducible canonical pathway for pyocin expression, so that RecA-dependent and RecA-independent pathways for pyocin expression are simultaneously active.…”

“…Does blocking recombinase activity activate the canonical or noncanonical pathways of pyocin expression? Further, elevated pyocin expression comes at a fitness cost, as Δ xerC strains grow more poorly than their wild-type counterparts and are more sensitive to antibiotics like ciprofloxacin ( 12 ). Hence, if XerC recombinase activity could be chemically inhibited to increase pyocin expression and sensitize cells to fluoroquinolone antibiotics, a recombinase inhibitor might have clinical utility when used in combination with fluoroquinolones.…”