With CysD forms the ATP sulfurylase (ATPS) that catalyzes the adenylation of sulfate producing adenosine 5'-phosphosulfate (APS) and diphosphate, the first enzymatic step in sulfur assimilation pathway. APS synthesis involves the formation of a high-energy phosphoric-sulfuric acid anhydride bond driven by GTP hydrolysis by CysN coupled to ATP hydrolysis by CysD. ATP + H(+) + sulfate = adenosine 5'-phosphosulfate + diphosphate Sulfur metabolism; hydrogen sulfide biosynthesis; sulfite from sulfate: step 1/3. Heterodimer composed of CysD, the smaller subunit, and CysN. Belongs to the TRAFAC class translation factor GTPase superfamily. Classic translation factor GTPase family. CysN/NodQ subfamily.