Type VI secretion system facilitates fitness, homeostasis, and competitive advantages for environmental adaptability and efficient nicotine biodegradation

ABSTRACT

Gram-negative bacteria employ secretion systems to translocate proteinaceous effectors from the cytoplasm to the extracellular milieu, thus interacting with the surrounding environment or micro-niche. It is known that bacteria can benefit from type VI secretion system (T6SS) by transporting ions to combat reactive oxygen species (ROS). Here, we report that T6SS activities conferred bacterial tolerance to nicotine-induced oxidative stress in Pseudomonas sp. strain JY-Q, a highly active nicotine degradation strain isolated from tobacco waste extract. AA098_13375 was identified to encode a dual-functional effector with anti-microbial and anti-ROS activities. Wild type strain JY-Q grew better than AA098_13375 deletion mutant in nicotine-containing medium by antagonizing increased intracellular ROS levels. It was, therefore, tentatively designated as TseN (Type VI Secretion system Effector for Nicotine tolerance), of which homologs were observed to be broadly ubiquitous in Pseudomonas species. TseN was identified as a Tse6-like bacteriostatic toxin via monitoring intracellular NAD⁺. TseN presented potential antagonism against ROS to fine tune the heavy traffic of nicotine metabolism in strain JY-Q. It is feasible that the dynamic tuning of NAD⁺ driven by TseN could satisfy demands from nicotine degradation with less cytotoxicity. In this scenario, T6SS involves a fascinating accommodation cascade that prompts constitutive biotransformation of N-heterocyclic aromatics by improving bacterial robustness/growth. In summary, T6SS in JY-Q mediated resistance to oxidative stress and promoted bacterial fitness via a contact-independent growth competitive advantage besides the well-studied T6SS-dependent antimicrobial activities.

IMPORTANCE

Mixtures of various pollutants and co-existence of numerous species of organisms are usually found in the adverse environments. Scientific community concerning biodegradation of nitrogen-heterocyclic contaminants commonly focused on screening functional enzymes transforming pollutants into intermediates of attenuated toxicity or for primary metabolism. Here we identified dual roles of T6SS effector TseN in Pseudomonas sp. JY-Q capable of degrading nicotine. T6SS could deliver TseN to kill competitors, and provide growth advantage by contact-independent pattern. TseN could monitor intracellular NAD⁺ level by its hydrolase activity, conferring cytotoxicity on competitive rivals but metabolic homeostasis on JY-Q. Moreover, JY-Q could be protected from TseN toxicity by its immunity protein TsiN. In conclusion, we found that TseN with cytotoxicity to bacterial competitors facilitated nicotine tolerance of JY-Q. We therefore revealed a working model between T6SS and nicotine metabolism. This finding indicates that multiple diversified weapons have been evolved by bacteria for their growth and robustness.