Molecular basis of beta-galactosidase alpha-complementation.

In previous studies, a cyanogen bromide peptide derived from amino-acid residues 3-92 of beta-galactosidase (EC 3.2.1.23; beta-D-galactoside galactohydrolase) was shown to have alpha-donor activity in intracistronic alpha-complementation. We have now isolated the defective beta-galactosidase alpha-acceptor protein from the deletion mutant strain M15 of Escherichia coli and find that it lacks residues 11-41 of betal-galactosidase. This is demonstrated by the isolation and sequence determination of a cyanogen bromide peptide from the M15 protein, which is identical to the corresponding peptide from beta-galactosidase except for the missing amino acids. We conclude that the alpha-donor peptide restores the region missing in the M15 protein.