The Pup-proteasome system regulates nitrate metabolism through an essential protein quality control pathway.

The human pathogen encodes a proteasome that carries out regulated degradation of bacterial proteins. It has been proposed that the proteasome contributes to nitrogen metabolism in , although this hypothesis had not been tested. Upon assessing growth in several nitrogen sources, we found that a mutant strain lacking the proteasomal activator Mpa was unable to use nitrate as a sole nitrogen source due to a specific failure in the pathway of nitrate reduction to ammonium. We found that the robust activity of the nitrite reductase complex NirBD depended on expression of the / chaperonin genes, which are regulated by the repressor HrcA. We identified HrcA as a likely proteasome substrate, and propose that the degradation of HrcA is required for the full expression of chaperonin genes. Furthermore, our data suggest that degradation of HrcA, along with numerous other proteasome substrates, is enhanced during growth in nitrate to facilitate the derepression of the chaperonin genes. Importantly, growth in nitrate is an example of a specific condition that reduces the steady-state levels of numerous proteasome substrates in .