Cloning, sequence and chromosomal location of a MEL gene from Saccharomyces carlsbergensis NCYC396.

Yeast strains producing alpha-galactosidase (alpha Gal) are able to use melibiose as a carbon source during growth or fermentation. We cloned a MEL gene from Saccharomyces carlsbergensis NCYC396 through hybridization to the MEL1 gene cloned earlier from Saccharomyces cerevisiae var. uvarum. The alpha Gal encoded by the newly cloned gene was galactose-inducible as is the alpha Gal encoded by MEL1. A probable GAL4-protein recognition sequence was found in the upstream region of the NCYC396 MEL gene. The gene was transcribed to a 1.5-kb mRNA which, according to the nucleotide sequence, encodes a protein of 471 amino acids (aa) with an Mr of 52,006. The first 18 aa fulfilled the criteria for the signal sequence, but lacked positively charged aa residues, except the initiating methionine. The enzyme activity was found exclusively in the cellular fraction of the cultures. The deduced aa sequence was compared to the aa sequences of other alpha Gal enzymes. It showed 83% identity with the S. cerevisiae enzyme, but only 35% with the plant enzyme, 30% with the human enzyme and 17% with the Escherichia coli enzyme. With pulsed-field electrophoresis, the MEL gene was located on chromosome X of S. carlsbergensis, whereas the S. cerevisiae var. uvarum MEL1 gene is located on chromosome II.