The tRNA methyltransferase TrmB is critical for Acinetobacter baumannii stress responses and pulmonary infection

Successful pathogens must be able to adapt to a multitude of stressors imposed by their host. Acinetobacter baumannii has emerged as a major global health threat due to its exceptional ability to adapt to hostile environments and skyrocketing rates of multidrug resistance. Recent studies have begun to explore the importance of tRNA methylation in the regulation of bacterial stress responses, including adaptation to antibiotic and oxidative stresses. However, tRNA methyltransferases (trms) have not been investigated in A. baumannii. Bioinformatic analyses revealed nine putative, SAM-dependent trms conserved across clinical A. baumannii isolates and laboratory strains. We generated eight trm mutants in a modern, colistin-resistant clinical isolate, ARC6851, and analyzed the mutants' stress responses. One mutant, Delta trmB, was vastly more sensitive to oxidative stress and displayed a growth defect at low pH. Accordingly, Delta trmB was unable to replicate in J774A.1 macrophages and had decreased virulence in an acute pneumonia murine model. Subsequently, we showed that A. baumannii TrmB makes the m7G tRNA modification. A proteomic analysis revealed that ARC6851 significantly upregulates a siderophore biosynthesis and uptake cluster, acinetobactin, under oxidative stress. In contrast, the upregulation of the acinetobactin proteins in Delta trmB was only modest, which impacted its ability to withstand iron deprivation under oxidative stress. qRT-PCR data showed that TrmB-dependent regulation of acinetobactin is post-transcriptional. Our results indicate that TrmB-mediated stress responses are important for A. baumannii pathogenesis, highlighting the therapeutic potential of targeting trms to combat the rise of multidrug-resistant A. baumannii. IMPORTANCE As deficiencies in tRNA modifications have been linked to human diseases such as cancer and diabetes, much research has focused on the modifica tions'tions' impacts on translational regulation in eukaryotes. However, the significance of tRNA modifications in bacterial physiology remains largely unexplored. In this paper, we demonstrate that the m7G tRNA methyltransferase TrmB is crucial for a top-priority pathogen, Acinetobacter baumannii, to respond to stressors encountered during infection, including oxidative stress, low pH, and iron deprivation. We show that loss of TrmB dramatically attenuates a murine pulmonary infection. Given the current efforts to use another tRNA methyltransferase, TrmD, as an antimicrobial therapeutic target, we propose that TrmB, and other tRNA methyltransferases, may also be viable options for drug development to combat multidrug-resistant A. baumannii.