A putative aminotransferase gene, cc3143 (aptA), from Caulobacter crescentus was screened by bioinformatical tools and overexpressed in E. coli, and the substrate specificity of the aminotransferase was investigated. AptA showed high activity for short-chain beta-amino acids. It showed the highest activity for 3-amino-n-butyric acid. It showed higher activity toward aromatic amines than aliphatic amines. The 3D model of the aminotransferase was constructed by homology modeling using a dialkylglycine decarboxylase PDB ID: 1DGE) as a template. Then, the aminotransferase was rationally redesigned to increase the activity for 3-amino- 3-phenylpropionic acid. The mutants N285A and V227G increased the relative activity for 3-amino-3-phenylpropionic acid to 3-amino-n-butyric acid by 11-fold and 3-fold, respectively, over that of wild type.