Soybean seeds are a major source of
protein, but contain low levels of
sulfur-containing
amino acids. With the objective of studying the
sulfur assimilation pathway of soybean, a full-length
cDNA clone for
5'-adenylylsulfate reductase (
APS reductase) was isolated and characterized. The
cDNA clone contained an open reading frame of 1414 bp encoding a 52 kDa
protein with a N-terminal chloroplast/plastid transit
peptide. Southern blot analysis of genomic
DNA indicated that the
APS reductase in soybean is encoded by a small multigene family. Biochemical characterization of the heterologously expressed and purified
protein shows that the clone encoded a functional
APS reductase. Although expressed in tissues throughout the plant, these analyses established an abundant expression of the gene and activity of the encoded
protein in the early developmental stages of soybean seed, which declined with seed maturity.
Sulfur and
phosphorus deprivation increased this expression level, while
nitrogen starvation repressed
APS reductase mRNA transcript and
protein levels. Cold-treatment increased expression and the total activity of
APS reductase in root tissues. This study provides insight into the
sulfur assimilation pathway of this nutritionally important legume.