Catalyzes the synthesis of S-sulfocysteine, utilizing O-phosphoserine (OPS) and thiosulfate as substrates. To a lesser extent, can also use sulfide as donor substrate, producing L-cysteine. CysK2 thus provides a third metabolic route to cysteine, either directly using sulfide as donor or indirectly via S-sulfocysteine. S-sulfocysteine might also act as a signaling molecule triggering additional responses in redox defense in the pathogen upon exposure to reactive oxygen species during intracellular survival or dormancy. Cannot utilize thiocarboxylated CysO as a sulfur donor and O-acetylserine (OAS) as acceptor substrate. O-phospho-L-serine + thiosulfate = phosphate + S-sulfo-L-cysteine H(+) + hydrogen sulfide + O-phospho-L-serine = L-cysteine + phosphate kcat is 12.1 min(-1) with O-phospho-L-serine and sulfide as substrates. kcat is 54.8 min(-1) with O-phospho-L-serine and thiosulfate as substrates. Amino-acid biosynthesis; L-cysteine biosynthesis. Homodimer. Was identified as a high-confidence drug target. The enzyme uses a mechanism via a central aminoacrylate intermediate that is similar to that of other members of this pyridoxal phosphate-dependent enzyme family. Belongs to the cysteine synthase/cystathionine beta-synthase family.