Figure 1. The pathway of cysteine biosynthesis. (a) The sulfate uptake and reduction arm of the pathway are shown on the left with the serine activation step in the middle and thiosulfate uptake on the right. (b) The sulfate-to-sulfide reduction branch of the pathway with the molecular structures of the sulfur containing intermediates drawn in ChemDraw accompanied by structures of the enzymes/transporter proteins (pdb codes 1sbp (sulfate binding protein), 1l7v (membrane components of the vitamin B₁₂ uptake system, which are representative of an ABC transporter), 1g8f (ATP sulfurylase), 1m7 h (APS kinase), 1sur (PAPS reductase), 5aop (sulfite reductase in which the atoms of the Fe₄S₄ cluster are represented as green spheres and the sirohaem moiety is shown in ball-and-stick)) drawn using MOLSCRIPT.

Figure 2. Regulation of cysteine biosynthesis in E. coli. (a) Scheme illustrating the components involved with the species that bind to CysB boxed. Feedback inhibition of serine acetyltransferase by cysteine blocks the accumulation of the inducer N-acetylserine (blue) that is required for CysB-mediated activation of the genes for sulfate reduction. Thiosulfate and sulfide (red) compete with N-acetylserine for binding to CysB and act as antiinducers. (b) CysB binding sites at various cys promoters. In all, 19 base-pair binding sites for CysB subunits are represented by boxes, their relative orientation indicated by arrows. The locations of these sites relative to the transcription start site are indicated below. (c) Ribbon drawing of the crystal structure of a dimeric cofactor-binding fragment of CysB (pdb code 1al3) with bound sulfate ions in ball-and-stick representation.