FIG. 5.

Model for the synthesis of nitrate reductase in T. thermophilus. A membrane NarCI and a soluble NarGHJ complex are initially formed. In the soluble complex, the NarG protein is inactive (squares). After attachment to the membrane, an inactive pentameric apo-nitrate reductase is formed. The enzyme is subsequently activated through the insertion of the Mo-bis-MGD cofactor (MGD) and the concomitant exit of the NarJ chaperone to render an active, heterotetrameric enzyme (circled NarG). Eventually, the active NarGH complex irreversibly separates from the membrane, giving rise to soluble forms that are active in nitrate reduction with artificial electron donors but not functional in nitrate respiration.