Oxygen-dependent partitioning of acetate assimilation via Acs and AckA-Pta pathways in Pseudomonas aeruginosa.
Watkins M E ME, Tonapi K V KV, VanDrisse C M CM
Pseudomonas aeruginosa is a metabolically versatile pathogen that thrives in host environments characterized by gradients of oxygen and nutrient availability. Although acetate is an abundant carbon source in chronic infections, the mechanisms governing P. aeruginosa acetate assimilation remains poorly understood. In many bacteria, acetate utilization occurs via the low-carbon-flux acetyl-CoA synthetase (Acs) or the high-carbon-flux acetate kinase/phosphotransacetylase (AckA-Pta) pathways. Here, we show that in P. aeruginosa, the above-mentioned pathways assimilate acetate as a function of oxygen availability, not as a function of carbon concentration. Our growth data demonstrate that AcsA is required for robust growth on acetate under oxic conditions, whereas the AckA-Pta pathway is essential for robust growth on acetate under anoxic conditions. Complementation experiments reveal that both pathways are functionally capable of acetate assimilation regardless of oxygen presence, indicating that pathway usage is primarily controlled by transcriptional regulation. Consistent with this, RT-qPCR analysis shows that acsA expression was elevated under oxic conditions, while ackA and pta were upregulated in the absence of oxygen. Despite these regulatory differences, kinetic analyses demonstrated that Acs and AckA from P. aeruginosa exhibit substrate affinities and catalytic efficiencies comparable to those of Salmonella enterica, indicating that the ability of both pathways to support P. aeruginosa growth across acetate concentrations is not due to altered enzyme kinetics like in other organisms. Together, these findings establish a new model in which oxygen availability, rather than acetate concentration, governs acetate assimilation in P. aeruginosa, with important implications for metabolic adaptation during infection. Recent work has highlighted metabolic regulation as a driver of virulence, yet how P. aeruginosa metabolizes key metabolites found at infection sites remains uncharacterized. P. aeruginosa has a strong preference for acetate over other conventional carbon sources such as glucose, and acetate is found at millimolar concentrations in hosts. However, how P. aeruginosa regulates pathways involved in acetate assimilation remains largely unknown. Because steep oxygen gradients exist in host niches, where P. aeruginosa encounters acetate, understanding how acetate assimilation pathways are regulated via oxygen tension is essential for identifying spatial vulnerabilities at infection sites.