Involved in the degradation of cholesterol. Catalyzes the introduction of a 9a-hydroxyl moiety into 1,4-androstadiene-3,17-dione (ADD) to yield the 9alpha-hydroxy-1,4-androstadiene-3,17-dione (9OHADD) intermediate which spontaneously form 3-hydroxy-9,10-seconandrost-1,3,5(10)-triene-9,17-dione (HSA) via the meta-cleavage of ring B with concomitant aromatization of ring A. androsta-1,4-diene-3,17-dione + 2 H(+) + O2 + 2 reduced [2Fe-2S]-[ferredoxin] = 9alpha-hydroxyandrosta-1,4-diene-3,17-dione + H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 1 [2Fe-2S] cluster per subunit. Binds 1 Fe cation. Lipid metabolism; steroid biosynthesis. Homotrimer. The two-component system 3-ketosteroid-9-alpha-monooxygenase is composed of an oxygenase component KshA and a reductase component KshB. Induced by KstR. Cells lacking this gene form 4-androstene-3,17-dione and 1,4-androstadiene-3,17-dione from sitosterol.