Studies of l-Cysteine Biosynthetic Enzymes in Phaseolus vulgaris L.

In higher plants the biosynthesis of l-cysteine from l-serine, acetylCoA, and sulfide requires serine transacetylase and O-acetylserine sulfhydrylase. The distribution of these enzymes in kidney bean (Phaseolus vulgaris L. cv. Red Kidney) seedlings was determined. Between one-third and two-thirds of the serine transacetylase activity was associated with mitochondria, whereas all of the O-acetyl-serine sulfhydrylase activity was present in the soluble fraction of cell homogenates. In a 14-day plant approximately two-thirds of the O-acetylserine sulfhydrylase activity and approximately one-half of the serine transacetylase activity was found in the leaves.Sulfur-deficient plants were grown to determine the effect of sulfur status on the levels of cysteine biosynthetic enzymes. Total extractable serine transacetylase activity was not affected by sulfur deficiency; in contrast, there was an increase in O-acetylserine sulfhydrylase activity under these conditions.