Catalyzes the oxidation of N-terminal cysteine residues (N-Cys), thus preparing the protein for N-end rule pathway-mediated proteasomal degradation, upstream of the N-end rule enzymes ATE1, ATE2 and PRT6 (PubMed:24599061, PubMed:29848548, PubMed:33207269). Controls the preparation of the group VII ethylene response factor (ERF-VII) proteins for degradation via the 26S proteasome N-end rule pathway (PubMed:24599061, PubMed:29848548, PubMed:33207269). Acts as an oxygen sensor that controls the stability of ERF-VII proteins, which are stabilized in flooding-induced hypoxia, and regulate transcriptional adaptation to these adverse conditions (Probable) (PubMed:29848548). Not active on Cys located inside or at the C-terminus of a peptide (PubMed:24599061). Acts redundantly with PCO1 to repress the anaerobic response (PubMed:24599061). L-cysteine + O2 = 3-sulfino-L-alanine + H(+) Binds 1 Fe(2+) cation per subunit. kcat is 1.98 sec(-1) with CGGAIISDFIPPPR peptide as substrate. Optimum pH is 8.5 with CGGAIISDFIPPPR peptide as substrate. Expressed throughout development, with the highest expression in mature siliques and during seed germination. Up-regulated by hypoxia (PubMed:20097791). Up-regulated by the ERF-VII transcription factor RAP2-12 during hypoxia. Belongs to the cysteine dioxygenase family. Truncated N-terminus.