Serine acetyltransferase.

Abstract

AbstractSulfur is an essential macronutrient for plants. Sulfate, the major form of inorganic sulfur, is reduced to sulfide by the assimilatory sulfate reduction pathway. Sulfide is integrated into organic compounds via cysteine biosynthesis. Serine acetyltransferase (SERAT, also called SAT; EC 2.3.1.30), which catalyses the formation of O-acetylserine (OAS) from serine and acetyl coenzyme A, is responsible for the entry step from glycine-serine metabolism to cysteine biosynthesis. OAS, as activated serine, is the direct precursor of cysteine formation catalysed by O-acetylserine (thiol)lyase (OASTL; EC 4.2.99.8). OASTL incorporates sulfide into OAS essentially by exchanging the alcohol group of serine (-CH-OH) against a thiol group (-CH-SH), thus producing cysteine. Both the SERAT and OASTL enzymes are encoded by members of large gene families and are localized in the cytosol, plastids and mitochondria. SERAT plays an important role in the regulation of sulfur assimilation and cysteine synthesis. The product of SERAT, OAS, is not only a limiting factor for cysteine synthesis but has also been suggested to be a signal molecule for gene expression in the sulfur assimilation pathway and to cause induction of a specific gene set, the OAS cluster genes. SERAT activity is modulated by two regulation mechanisms: feedback inhibition by cysteine and the formation of a protein complex with OASTL, which allows the regulation of OAS and cysteine biosynthesis in plant cells. Analysis of SERATs using various approaches in recent years has provided substantially new information concerning the properties and functions of these enzymes in sulfur metabolism. Here we review the available data on SERAT function in Arabidopsis.