In organisms like Archaeoglobus fulgidus lacking EC 6.1.1.16 (cysteine¡ªtRNA ligase) for the direct Cys-tRNACys formation, Cys-tRNACys is produced by an indirect pathway, in which EC 6.1.1.27 (O-phosphoseryl¡ªtRNA ligase) ligates O-phosphoserine to tRNACys, and EC 2.5.1.73 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¡ªtRNA ligase) and the indirect pathway with EC 6.1.1.27 (O-phosphoseryl-tRNA ligase) and EC 2.5.1.73 [2].
