The xylFGHR operon in E. coli contains four genes involved in the transport of xylose into the cell, and the regulation of the operon. This operon is both positively and negatively regulated. It is negatively regulated by the DNA-binding protein Fis, which binds to the promoter region and represses transcription of the operon. The operon can also be activated by the cAMP-activated global transcriptional regulator CRP, which is itself activated by the binding of cAMP. cAMP generally has a higher cellular concentration when glucose is not being used as the primary energy source of the cell, and in such conditions, alternative sources of carbon such as xylose would be needed. When this complex binds to the promoter region, it activates transcription of the operon, leading to increased levels of xylose transport into the cell. The operon can also be activated by the xylose operon regulatory protein, which is activated by the binding of D-xylose. This also binds to and activates transcription of the operon. The first gene in the operon, xylF, encodes a D-xylose-binding periplasmic protein, which exists in the periplasm of the cell and binds to D-xylose. It is a part of the xylose transport system, along with the proteins encoded by xylG and xylH that are responsible for moving xylose against the concentration gradient into the cell. The second gene, xylG, encodes the xylose import ATP-binding protein XylG, which is responsible for providing the energy necessary for the transport of xylose across the membrane and concentration gradient. The third gene, xylH, encodes the xylose transport system permease protein xylH, which is likely responsible for moving xylose molecules across the cell's inner membrane. The final gene in the operon, xylR, encodes the xylose operon regulatory protein, which regulates this operon, as well as tye xylAB operon, which is involved with the breakdown of xylose.

Operon: Xylose Transport References