Regulation of ATP-sulfurylase by various nitrogen and sulfur sources in cultured Ipomoea sp.

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Abstract
ATP-sulfurylase of Ipomoea sp. (morning glory) grown in suspension culture on a chemically defined medium is incompletely repressed by a readily assimilated sulfur source such as sulfate. The developmental pattern of the enzyme shows two peaks of activity, one peak before the onset and the other at the end of the logarithmic phase of growth. However, when ammonium plus nitrate is replaced by ammonium or casein hydrolysate as a nitrogen source in the medium, one peak of enzyme activity is observed. The enzyme is derepressed during growth of the cells on a slowly assimilated sulfur source such as L-djenkolate or cysteic acid or under conditions of sulfur starvation. Cysteic acid derepresses the enzyme even in the presence of repressible levels of sulfate in the medium. The enzyme also is derepressed when Ipomoea is grown on cysteine or methionine at a concentration of sulfur equivalent to that present in 1 mM sulfate. These amino acids repress ATP-sulfurylase only when added to the medium containing derepressing levels of sulfate (0.1 mM). Cells grown on a slowly assimilated nitrogen source such as proline, histidine, or nitrate form lower levels of ATP-sulfurylase than cells grown on readily assimilated nitrogen such as ammonium, ammonium plus nitrate, or casein hydrolysate. Thus, sulfate assimilation is subject to positive control by nitrogen metabolism and negative control by sulfur sources such as cysteine and methionine.