Cysteine sulfate uptake, regulation

Fig. 3. Regulation of the bound pathway for the assimilation of sulfate into cysteine and associated processes. Carrier refers to an endogenous thiol of uncertain identity in higher plants. Enzymes associated with the sulfate assimilation pathway and the synthesis of O-acetylseiine are (1) high-ailinity sulfate uptake mechanism, (2) ATP-sulfurylase, (3) adenosine S -phosphosulfate (APS) sulfotransferase, (4) organic thiosulfate reductase, (5) cysteine synthase, and (6) serine transacetylase. Cysteine sulfhydrase (7), an enzyme of cysteine catabolism, and nitrate reductase (8), the first enzyme of the nitrate assimilation pathway, are also shown. Inhibitory control of the pathways is shown by discontinuous lines (----) and enhancement by continuous lines (------).

S.a. is regulated by enzyme induction and repression, and by feedback inhibition (Fig. 2). In E. coli, l-cysteine represses all steps of S.a. (assuming S.a. to proceed via free intermediates) starting with the uptake of sulfate by the cell. O-Acetyl-t-serine induces uptake of sulfate, synthesis of APS and PAPS and synthesis of sulfite reductase, but not of sulfate reductase. Acetylation of L-serine to O-acetyl-L-serine is feedback-inhibited by L-cysteine. 

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