| Transporter or enzymes | Microorganisms | |
| LDa | cpLN | |
| CitP | citQRP operon harboured by a 8 kb plasmidConstituvely expressed and enhanced under acidic conditions (pH 4.5) [16] [22] [23] . | citI-citMCDEFGRP cluster harboured by a 22 kb plasmidLow constitutive transcription, enhanced by citrate addition [20] [21] . |
| CL and OAD | Chromosomal citM-citI-citCDEFXG clusterConstitutively expressed, and enhanced under acidic conditions (pH 4.5 to 5) [1] [16] [24] [25] . | |