The functional mutational landscape of the lacZ gene

The lacZ gene of Escherichia coli encodes beta-galactosidase (beta-gal), a lactose metabolism enzyme of the lactose operon. Previous chemical modification or site-directed mutagenesis experiments have identified 21 amino acids that are essential for beta-gal catalytic activity. We have assembled over 10,000 lacZ mutations from published studies that were collected using a positive selection assay to identify mutations in lacZ that disrupted beta-gal function. We analyzed 6,465 independent lacZ mutations that resulted in 2,732 missense mutations that impaired beta-gal function. Those mutations affected 492 of the 1,023 lacZ codons, including most of the 21 previously known residues critical for catalytic activity. Most missense mutations occurred near the catalytic site and in regions important for subunit tetramerization. Overall, our work provides a comprehensive and detailed map of the amino acid residues affecting the structure and catalytic activity of the beta-gal enzyme.