Abstract: A procedure for producing a modified 7.beta.-(4-carboxybutanamide) cephalosporinase enzyme which can be purified in a single chromatographic step. The procedure for production of the enzyme involves: mutagenizing the gene which codes for the enzyme from Acinetobacter sp. ATCC 53891 by inserting a nucleotide sequence coding for six histidine residues; fusing the mutant gene with high-efficiency promoter DNA sequences; transforming Escherichia coli cells with the fusion gene construct; growing the transformed Escherichia coli cells; and recovering the enzyme by the use of supports which contain metal chelates. 7-Aminocephalosporanic acid is an important intermediate for the manufacture of a wide range of antibacterial agents of the cephalosporin family.