Adaptive laboratory evolution optimizes an engineered phosphite utilization pathway in Synechococcus elongatus PCC...

Synthetic biology approaches enable the creation of promising chassis for practical application in various fields, though engineering of microbial metabolism often imposes a metabolic burden, potentially driving adaptive evolution during long-term cultivation. A previously established phosphite (Pt)-dependent metabolic system has proven to be an effective strategy for the containment of genetically engineered microorganisms, although its implementation accompanied a slight growth retardation. Here, we investigated the effect of long-term serial passaging cultivation on the Pt-dependent strain of Synechococcus elongatus PCC 7942, RH714. Compared with the originally constructed RH714, the passaged population of RH714 exhibited improved growth and a higher rate of Pt consumption in culture medium. Sequence analysis revealed point mutations within the introduced htxBCDE transporter genes, which are required for selective incorporation of Pt as a phosphorus nutrition. Introduction of the mutated gene cluster into S. elongatus PCC 7942 reproduced the traits of the passaged RH714 population, suggesting that these genetic changes enhance Pt transport activity and account for the observed phenotypes. Disruption of endogenous phosphate (Pi) transporter genes in the strains expressing the mutated htxBCDE-ptxD cluster abolished growth in Pi-containing medium, suggesting that the mutations in the transporter genes did not alter substrate specificity toward Pi. These results indicated that long-term passage cultivation developed an optimized mutant capable of efficient proliferation under the Pt metabolizing conditions without compromising its biocontainment capability.