Horizontal transfer of a pathway for coumarate catabolism unexpectedly inhibits purine nucleotide biosynthesis

Metabolic pathways are frequently transferred between bacterial strains in the environment through horizontal gene transfer (HGT), yet laboratory engineering to introduce new metabolic pathways often fails. Successful use of a pathway requires co-evolution of both pathway and host, and these interactions may be disrupted upon transfer to a new host. Here we show that two different pathways for catabolism of coumarate failed to function when initially transferred into Escherichia coli. Using laboratory evolution, we elucidated the factors limiting activity of the newly-acquired pathways and the modifications required to overcome these limitations. Both pathways required mutations to the host to enable effective growth with coumarate, but the necessary mutations differed depending on the chemistry and intermediates of the pathways. In one case, an intermediate inhibited purine nucleotide biosynthesis, and this inhibition was relieved by single amino acid mutations to IMP dehydrogenase. A strain that natively contains this coumarate catabolism pathway, Acinetobacter baumannii, is already resistant to inhibition by the relevant intermediate, suggesting that natural pathway transfers have faced and overcome similar challenges. These discoveries will aid in our understanding of HGT and ability to predictably engineer metabolism.