In higher plants, multiple copies of the cysteine synthase gene are present for cysteine biosynthesis. Some of these genes also have the potential to produce various kinds of beta-substitute alanine. In the present study, we cloned a 1275-bp cDNA for cytosolic O-acetylserine(thiol)lyase (cysteine synthase) (Cy-OASTL) from Leucaena leucocephala. The purified protein product showed a dual function of cysteine and mimosine synthesis. Kinetics studies showed pH optima of 7.5 and 8.0, while temperature optima of 40 and 35 degrees C, respectively, for cysteine and mimosine synthesis. The kinetic parameters such as apparent K-m, k(cat) were determined for both cysteine and mimosine synthesis with substrates O-acetylserine (OAS) and Na2S or 3-hydroxy-4-pyridone (3H4P). From the in vitro results with the common substrate OAS, the apparent k(cat) for Cys production is over sixfold higher than mimosine synthesis and the apparent K-m is 3.7 times lower, suggesting Cys synthesis is the favored pathway.