Core and auxiliary functions of one-carbon metabolism in <i>Pseudomonas putida</i> exposed by a systems-level analysis of transcriptional and physiological responses

Abstract

The soil bacterium <i>Pseudomonas putida</i> is a robust biomanufacturing host that assimilates a broad range of substrates while efficiently coping with adverse environmental conditions. <i>P. putida</i> is equipped with functions related to one-carbon (C1) compounds (e.g. methanol, formaldehyde, and formate) oxidation-yet pathways to assimilate these carbon sources are largely absent. In this work, we adopted a systems-level approach to study the genetic and molecular basis of C1 metabolism in <i>P. putida</i>. RNA sequencing identified two oxidoreductases, encoded by <i>PP_0256</i> and <i>PP_4596</i>, transcriptionally active in the presence of formate. Quantitative physiology of deletion mutants revealed growth defects at high formate concentrations, pointing to an important role of these oxidoreductases in C1 tolerance. Moreover, we describe a concerted detoxification process for methanol and formaldehyde, the C1 intermediates upstream formate. Alcohol oxidation to highly-reactive formaldehyde by PedEH and other broad-substrate-range dehydrogenases underpinned the (apparent) suboptimal methanol tolerance of <i>P. putida</i>. Formaldehyde was mostly processed by a glutathione-dependent mechanism encoded in the <i>frmAC</i> operon, and thiol-independent FdhAB and AldB-II overtook detoxification at high aldehyde concentrations. Deletion strains were constructed and characterized towards unveiling these biochemical mechanisms, underscoring the worth of <i>P. putida</i> for emergent biotechnological applications-e.g. engineering synthetic formatotrophy and methylotrophy. IMPORTANCE C1 substrates continue to attract interest in biotechnology, as their use is both cost-effective and ultimately expected to mitigate the impact of greenhouse gas emissions. However, our current understanding of bacterial C1 metabolism remains relatively limited in species that cannot grow on (i.e., assimilate) these substrates. <i>Pseudomonas putida</i>, a model Gram-negative environmental bacterium, constitutes a prime example of this sort. The biochemical pathways active in response to methanol, formaldehyde, and formate have been largely overlooked-although the ability of <i>P. putida</i> to process C1 molecules has been previously alluded to in the literature. By using a systems-level strategy, this study bridges such knowledge gap through the identification and characterization of mechanisms underlying methanol, formaldehyde, and formate detoxification-including hitherto unknown enzymes that act on these substrates. The results reported herein both expand our understanding of microbial metabolism and lay a solid foundation for engineering efforts toward valorizing C1 feedstocks.