A truncated mutant alpha-amylase, Xa-S2, was obtained from Xanthomonas campestris wild type alpha-amylases (Xa-WT) through random mutagenesis that contained 167 amino acid residues (approx 65% shorter than that of Xa-WT). Secondary structure prediction implied that Xa-S2, would be unable to form the whole (beta/alpha)(8)-barrel catalytic domain and did not have the three conserved catalytic residues of wild type alpha-amylase, but it still displays the starch-hydrolyzing activity. Xa-S2 was prepared, characterized and compared to the recombinant wild-type enzymes. The K-m for starch was 32 mg/ml; activity was optimal at pH 6.2 and 30 degrees C. In contrast, the K-m for starch of Xa-WT was 8 mg/ml and optimal enzyme activity was at pH 6.0-6.2 and 45-50 degrees C. Our results suggested that Xa-S2 is a new amylase with a minimal catalytic domain for hydrolyzing substrates with of alpha-1,4-glucosidic bonds.