A rapid method for the purification of large amounts of an alpha-complementing peptide derived from beta-galactosidase (E. coli).

A DNA segment coding for residues 6-44 of beta-galactosidase was ligated to a vector with the glutathione-S-transferase gene which also contained a sequence coding for a thrombin recognition site. The fused protein, with an additional 9 amino acids coded for by the vector, was purified using a glutathione agarose affinity column. A peptide made up of residues 6-44 of beta-galactosidase and the 9 additional amino acids was then cleaved from the glutathione-S-transferase using thrombin and purified with a gel filtration column. The peptide was about 3-4 times as active for alpha-complementation as the alpha-peptide derived from CNBr digestion of wild type beta-galactosidase.