Sequence of the Kluyveromyces lactis β-galactosidase: comparison with prokaryotic enzymes and secondary structure analysis

The LAC4 gene encoding the β-galactosidase (βGal) of the yeast, Kluyveromyces lactis, was cloned on a 7.2-kb fragment by complementation of a lacZ-deficient Escherichia coli strain. The nucleotide sequence of the structural gene, with 42 bp and 583 bp of the 5'- and 3'-flanking sequences, respectively, was determined. The deduced amino acid (aa) sequence of the K. lactis βGal predicts a 1025-aa polypeptide with a calculated M r. of 117618 and reveals extended sequence homologies with all the published prokaryotic βGal sequences. This suggests that the eukaryotic βGal is closely related, evolutionarily and structurally, to the prokaryotic βGal's. In addition, sequence similarities were observed between the highly conserved N-terminal two-thirds of the βGal and the entire length of the β-glucuronidase (βGlu) polypeptides, which suggests that βGlu is clearly related, structurally and evolutionarily, to the N-terminal two-thirds of the βGal. The structural analysis of the βGal alignment, performed by mean secondary structure prediction, revealed that most of the invariant residues are located in turn or loop structures. The location of the invariant residues is discussed with respect to their accessibility and their possible involvement in the catalytic process.