In organisms like Archaeoglobus fulgidus lacking EC
6.1.1.16 (cysteine---tRNA ligase) for the direct Cys-tRNACys formation, Cys-tRNACys is produced by an indirect pathway, in which EC
6.1.1.27 (O-phosphoseryl-tRNA ligase) ligates O-phosphoserine to tRNACys, and EC
2.5.1.73 (O-phospho-L-seryl-tRNA: Cys-tRNA synthase) converts the produced O-phospho-L-seryl-tRNACys to Cys-tRNACys. The SepRS/SepCysS pathway is the sole route for cysteine biosynthesis in the organism [1]. Methanosarcina mazei can use both pathways, the direct route using EC
6.1.1.16 (cysteine---tRNA ligase) and the indirect pathway with EC
6.1.1.27 and EC
2.5.1.73 (O-phospho-L-seryl-tRNA: Cys-tRNA synthase) [2].