Catalyzes the reversible reduction of 6-hydroxynicotinate to 6-oxo-1,4,5,6-tetrahydronicotinate. 1,4,5,6-tetrahydro-6-oxonicotinate + oxidized 2[4Fe-4S]-[ferredoxin] = 6-hydroxynicotinate + 2 H(+) + reduced 2[4Fe-4S]-[ferredoxin] Binds 1 flavin covalently per subunit. Binds 1 [2Fe-2S] cluster per subunit. Binds 2 [4Fe-4S] clusters per subunit. Optimum pH for the reduction of 6-hydroxynicotinate is 6.5. Activity declines rapidly below pH 5.6. Cofactor degradation; nicotinate degradation; propanoate and pyruvate from 6-hydroxynicotinate: step 1/8. Homotetramer.