Intracellular folic acid concentration. Folic acid concentration in E. coli BL21(DE3)ΔabgTΔpabA double knockout cells expressing YdaH were markedly reduced in comparison with cells transformed with the empty vector. When transformed with plasmid expressing the mutant transporter, D180A, N390, W400A, P418A, D429A and N433A, folic acid production was significantly increased in these cells. However, the level of intracellular folic acid concentration in BL21(DE3)ΔabgTΔpabA cells expressing R426A was nearly identical to that of the double knockout strain carrying wild-type YdaH. Error bars denote standard deviation of biological replicates, n = 4. “*” indicates values of BL21(DE3)ΔabgTΔpabA/pET15b and BL21(DE3)ΔabgTΔpabA cells expressing the mutant transporters that are significantly higher than that of BL21(DE3)ΔabgTΔpabA/pET15bΩydaH expressing wild-type YdaH (P < 0.02; student’s t-test).