The caiTABCDE operon in E. coli contains six genes that produce proteins involved in the anaerobic metabolism of carnitine. This operon can be activated by the aerobic respiration control protein ArcA, which binds to several places upstream of the promoter. This binding activates transcription in the proper conditions, such as the need for fermentative metabolism. The operon can also be repressed by the DNA-binding protein H-NS. This protein binds to and alters the structure of DNA, suppressing the genes until it is removed. Finally, the caiA gene mRNA can specifically be prevented from being translated by small regulatory RNA that binds to the transcript, while the other genes are translated normally. The first gene in the operon, caiT, encodes for the L-carnitine/gamma butyrobetaine antiporter protein, which is responsible for transporting methyl-L-carninte into the cell, while transporting gamma butyrobetaine out of it. The second gene, caiA, encodes crotonobetainyl-CoA reductase, a protein that catalyzes the conversion of crotonobetainyl-CoA into gamma-butyrobetainyl-CoA. This protein is thought to interact with L-carnitine CoA-transferase, the protein encoded by caiB, which transfers the CoA from gamma-butyrobetainyl-CoA to L-carnitine, forming L-carnityl-CoA and gamma-butyrobetaine. It can also transfer between crotonobetainyl-CoA and L-carnitine. The fourth gene in the operon, caiC, encodes a crotonobetaine/carnitine CoA ligase, which like caiB catalyzes the reversible transfer of CoA between carnitine, gamma-butyrobetaine and crotobetaine. The fifth gene, caiD, encodes carnitinyl-CoA dehydratase, which reversibly removes a water molecule from crotonobetainyl-CoA, forming carnitinyl-CoA. The final gene, caiE, encodes a putative transferase whose function is currently unknown. However, when the protein is overexpressed, it functions similarly to caiB and caiD, as a CoA ligase and dehydratase.

Operon: Carnitine Operon References