Dioxygenase that catalyzes the cleavage of the aromatic ring of 5-nitrosalicylate (5NSA) without prior removal of the nitro group in biodegradation of 5-nitroanthranilate. 5-nitrosalicylate + O2 = 2-oxo-3-(5-oxofuran-2-ylidene)propanoate + H(+) + nitrite