Cultural assimilator Subject

Thus, they concluded that ATP-sulfurylase was subject to feed-forward stimulation by a reaction product of nitrate reduction and that nitrate reductase was subject to an analogous feed forward by a product of sulfate, thus coordinating the first step of each of the two pathways. Some support for this mechanism has come fi om experiments with cell cultures of Ipomea (Zink, 1984) and from measurements of ATP-sulfurylase and nitrate reductase in the leaves of whole tobacco plants (Barney and Bush, 1985), though in the latter study the response of ATP-sulfurylase to nitrogen stress was far less pronounced than the response of nitrate reductase to sulfur stress. However, as discussed below, APS sulfo-transferase rather than ATP-sulfurylase appears to be the principal determining site for the feed-forward control of sulfate assimilation by nitrogen assimilation products in Lemna. 

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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