Lemna cysteine synthesis

Most of the inorganic sulfate assimilated and reduced by plants appears ultimately in cysteine and methionine. These amino acids contain about 90% of the total sulfur in most plants (Allaway and Thompson, 1966). Nearly all of the cysteine and methionine is in protein. The typical dominance of protein cysteine and protein methionine in the total organic sulfur is illustrated in Table I by analyses of the sulfur components of a lower plant (Chlorella) and a higher plant (Lemna). Thede novo synthesis of cysteine and methionine is one of the key reactions in biology, comparable in importance to the reduction of carbon in photosynthesis (Allaway, 1970). This is so because all nonruminant animals studied require a dietary source of methionine or its precursor, homocysteine. Animals metabolize methionine via cysteine to inorganic sulfate. Plants complete the cycle of sulfur by reduction of inorganic sulfate back to cysteine and methionine, and are thus the ultimate source of the methionine in most animal diets (Siegel, 1975). 

The level of sulfotransferase in Lemna and in Phaseolus vulgaris is also subject to strong inhibition by gaseousH2S(Brunoldand Schmidt, 1976,1978 Wyss and Brunold, 1979). However, the extractable acti vity of cysteine synthase is not similarly affected. Removal of H2S firom the gas phase results in rapid restoration of activity which, based on a study of labeling of the enzyme (von Arb and Brunold, 1980), was attributed to synthesis ofthe enzyme de novo. HjS also inhibits the level of APS sulfotransferase in cell suspension cultures of Nicotiana sylvestris in this tissue neither the ATP-sulfiirylase or cysteine synthase activity was affected by H2S or cysteine (Brunold etal., 9Sl). Importantly, the inhibition of APS sulfotransferase by H2S was correlated with an enhanced level of cysteine, suggesting that the H2S inhibition could have been mediated via this reaction product. Uptake of exogenous sulfate was also inhibited by H2S in this system (Brunold et al., 1981). 

In tobacco cell cultures the extractable levels of ATP-sulfurylase and cysteine synthase are very low when the cells are subject to nitrogen stress but increase rapidly upon alleviation of the stress, suggesting that a product of nitrogen assimilation derepresses the levels of these enzymes. In Lemna (and possibly in cultured Rosa cells) it appears that this role is fulfilled by APS sulfotransferase and that ATP-sulfurylase and cysteine play unimportant roles in coordinating the sulfate assimilation pathway with the nitrate assimilation pathway. A further regulatory mechanism known to occur in cultured tobacco cells is that excessively high concentrations of cysteine induce the synthesis of cysteine desulfiiydrase (see Section VI). 

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