Isomerization of pyrogallol to phloroglucin. 1,2,3,5-tetrahydroxybenzene + 1,2,3-trihydroxybenzene = 1,2,3,5-tetrahydroxybenzene + 1,3,5-trihydroxybenzene Binds 3 [4Fe-4S] clusters. Heterodimer of a large and a small subunit.