Also known as MHPC_BURVG, mhpC. Catalyzes the cleavage of the C5-C6 bond of 2-hydroxy-6-oxononadienedioate and 2-hydroxy-6-oxononatrienedioate, a dienol ring fission product of the bacterial meta-cleavage pathway for degradation of phenylpropionic acid. Homodimer.