FIG. 6.

Multiple functions and regulation of PutA. (A) Reactions catalyzed by PutA. (B) Overall model of PutA functional switching. In the absence of proline, the flavin cofactor is oxidized and PutA accumulates on the DNA, repressing expression of the put genes. The five PutA binding sites are labeled as O1, O2, O3, O4, and O5. The black arrows represent the β-strands of the RHH domain that binds the DNA. When proline is available, the flavin cofactor is reduced by PutA, resulting in activation of PutA-membrane binding. The change in intracellular location of PutA in response to proline relieves repression of the put regulon.