Sulfate assimilation regulation

Smith, I.K. Regulation of sulfate assimilation in tobacco cells Plant Physiol. 66 (1980) 877-883. 

The various APS and PAPS hydrolases described in Secticm III could be of some significance in the regulation of sulfate assimilation. However, since the synthesis of APS and PAPS are known to occur in chloroplasts, these possibilities must remain speculative until the subcellular localization of the enzymes catalyzing Eqs. (4)-(6) are established especially since Burnell and Anderson (1973a) reported that nonspecific 3 -nucleotidase is not associated with chloroplasts. Recently, Sawhney and Nicholas (1976b) have reported that particulate fractions from Anabaena degrade PAPS to sulfate. ADP sul-furylase, which catalyzes the reaction... 

These data indicate that methionine, or a derivative thereof, controls vivo assimilation of sulfate into cystathionine and its products, and therefore that the regulatory locus is at cystathionine synthesis. Furthermore, since regulation at this step did not cause an accumulation of cysteine and its products, regulation of sulfate assimilation into cysteine is also indicated. It has not yet been firmly established whether methionine also controls novo synthesis of the carbon moieties of methionine. Such regulation of the 4-carbon moiety would be expected if exogenous methionine regulates the cystathionine synthesis step, since it is at this step that both the sulfur and 4-carbon moieties become committed to methionine. 

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 (------).

B. Regulation of the Sulfate Assimilation Pathway 1. Internal Regulation... 

The internal regulation of the sulfate assimilation pathway and its coordination with the nitrate assimilation pathway are summarized in Fig. 3. It shows that cysteine is a negative effector of serine transacetylase and that it also controls the level of APS sulfotransferase. The inhibitory effects of HjS on the level of APS sulfotransferase are probably mediated via cysteine, though HjS itself at high concentrations inhibits cysteine synthase. 

Cysteine synthase is inhibited by high concentrations of cysteine and of a number of other amino acids in the sulfur assimilation pathway (Ng and Anderson, 1978 Ascaho and Nicholas, 1977), but the high concentrations required cast doubt on the physiological significance of these inhibitions. The amounts of cysteine synthase were comparable in kidney beans growing either under conditions of sulfur deficiency or adequate sulfate nutrition (Smith, 1972), and in L. minor growing either in the presence of sulfate as a sole source of sulfur, or supplemented with HjS (Brunold and Schmidt, 1976) or cysteine (Brunold and Schmidt, 1978). Such studies further indicate that the amounts of cysteine synthase may not be regulated appreciably by repression and derepression. 

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