The codBA operon in E. coli contains two genes which encode proteins involved in cytosine entry into the cell, as well as its deamination. The operon can be activated by the bining of nitrogen assimilation regulatory protein Nac, which activates transcription at the promoter in the case of low nitrogen levels in the cell. With this operon activated, cytosine can be broken down into the nitrogen source, and can even be the sole nitrogen source if necessary. The operon can also be deactivated by the binding of the PurR transcriptional repressor to the promoter. This repressor is activated by a purine such as guanine, or its derivatives such as hypoxanthine. The first gene in the operon, codB, encodes cytosine permease, an enzyme that is necessary for cytosine to be able to enter the cell. The second gene, codA, encodes cytosine deaminase, an enzyme that catalyzes the deamination of cytosine into uracil, with the addition of a water molecule.

Operon: Cytosine Transport References