Inactivation of the Lactococcus lactis high-affinity phosphate transporter confers oxygen and thiol resistance and alters metal homeostasis

Abstract

Numerous strategies allowing bacteria to detect and respond to oxidative conditions depend on the cell redox state. Here we examined the ability ofLactococcus lactisto survive aerobically in the presence of the reducing agent dithiothreitol (DTT), which would be expected to modify the cell redox state and disable the oxidative stress response. DTT inhibitedL. lactisgrowth at 37 °C in aerobic conditions, but not in anaerobiosis. Mutants selected as DTT resistant all mapped to thepstFEDCBAlocus, encoding a high-affinity phosphate transporter. Transcription ofpstFEDCBAand a downstream putative regulator of stress response,phoU, was deregulated in apstAstrain, but amounts of major oxidative stress proteins were unchanged. As metals participate in oxygen radical formation, we compared metal sensitivity of wild-type andpstAstrains. ThepstAmutant showed approximately 100-fold increased resistance to copper and zinc. Furthermore, copper or zinc addition exacerbated the sensitivity of a wild-typeL. lactisstrain to DTT. Inactivation ofpstAconferred a more general resistance to oxidative stress, alleviating the oxygen- and thermo-sensitivity of aclpPmutant. This study establishes a role for thepstlocus in metal homeostasis, suggesting thatpstinactivation lowers intracellular reactivity of copper and zinc, which would limit bacterial sensitivity to oxygen.