The appCBXA operon in E. coli contains four genes that encode for cytochrome bd-II subunits, as well as an acid phosphatase. The operon is activated in situations with low oxygen by the binding of RNA polymerase sigma factor RpoS. This sigma factor allows RNA polymerase to bind, transcribing the operon. The operon can also be activated by the HTH-type transcriptional regulator AppY, which activates the translation and synthesis of these proteins during growth phase deceleration. It can also stabilize RpoS if the bacteria is stressed. The operon can be inactivated by the binding of cAMP-activated global transcriptional regulator CRP (CAP). This complex is activated when cAMP levels are high in the cell, and binds to the promoter region, preventing RNA polymerase from transcribing the operon. The first two genes in the operon, appC and appB, encode the cytochrome bd-II oxidase subunits 1 and 2, which form a heterodimer. The third gene, appX, encodes a putative cytochrome bd-II ubiquinol oxidase subunit, which may be part of a bd-II oxidase trimer along with the products of appC and appB. The fourth gene, appA, produces phosphoanhydride phosphorylase, previously referred to as the pH 2.5 acid phosphatase. This enzyme removes a phosphate group from a molecule, and works optimally at a pH of 2.5. It also has its own promoter, and can be transcribed without the rest of the operon.

Operon: Cytochrome bd Terminal Oxidase appCBXA References