Analysis of the Kluyveromyces lactis positive regulatory gene LAC9 reveals functional homology to, but sequence divergence from, the Saccharomyces cerevisiae GALA gene

The galactose metabolism positive regulatory gene from Kluyveromyces lactis, LAC9, has been isolated through its ability to activate expression of galactose metabolism enzyme genes in Saccharomyces cerevisiae. The LAC9 gene also activates expression of the S. cerevisiae a-galactosidase (MELl) and K. lactis β-galactosidase (LAC4) genes in S. cerevisiae. Although LAC9-activated gene expression in K. lactis is not glucose repressed, activation of MELl gene expression by LAC9 in S. cerevisiae is. The LAC9 gene is expressed at an extremely low level as a 2.9-kb mRNA, and encodes a protein of 865 amino acids. Although the LAC9 gene is functionally analogous to the, S. cerevisiae GALA gene, the bulk of its protein sequence shows little homology to that of GALA. Two of the three regions of homology that do exist, however, are restricted to areas of GAL4 protein already implicated in nuclear localization, DNA binding, and transcriptional activation.