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 (Probable) (PubMed:29848548, PubMed:33207269, PubMed:32868422). Controls the preparation of the group VII ethylene response factor (ERF-VII) proteins for degradation via the 26S proteasome N-end rule pathway (Probable) (PubMed:29848548, PubMed:33207269, PubMed:32868422). 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). L-cysteine + O2 = 3-sulfino-L-alanine + H(+) Binds 1 Fe(2+) cation per subunit. kcat is 7.9 sec(-1) with CGGAIISDFIPPPR peptide as substrate. Optimum pH is 9.0 with CGGAIISDFIPPPR peptide as substrate. Belongs to the cysteine dioxygenase family.