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The gene HIS4 from Kluyveromyces lactis is transcriptionally activated in complete synthetic respect to rich media and in an independent mechanism related to carbon source. This regulation was not previously described for Saccharomyces cerevisiae HIS4. The EMSA assay carried out with F7 showed a specific band, Fc1, in YPG, and two bands, Fc2 and Fc3, in complete medium. The Fc2 and Fc3 bands were dependent on the carbon source present in the medium, since their intensities were higher in glycerol than in glucose. The protein or proteins causing the Fc1 band seem to be involved in the different regulation mechanisms between rich and synthetic complete media because the Fc1 band was detected in cells grown in synthetic medium. Therefore, the promoter region (-200 to -173) is responsible for two independent regulatory mechanisms.

Two genomic fragments containing the KlADE2 and the KlPUT2 genes were isolated from Kluyveromyces lactis genomic library by functional complementation of the corresponding ade2 and put2 mutations in Saccharomyces cerevisiae. The deduced KlAde2p and KlPut2p amino acid sequences displayed strong similarities to their counterparts in Saccharomyces cerevisiae (75% and 72% identity respectively) with highly conserved domains. Thus, we report the isolation and characterization of ADE2 and PUT2 genes from Kluyveromyces lactis, including their protein structure, flanking sequence regions, and transcriptional gene regulation by different nutrients in the medium.