From Genome to Life:

SIBIRNA Kateryna

Institut de Génétique et Microbiologie -, Université Paris Sud -, 91405 Orsay, France

title: Functional evolution of regulatory networks : search for the orthologs of the glucose-repressed regulated subunit (Hap4p) of the HAP transcriptional complex in hemiascomycetes yeasts.

K. Sybirna, W. Bao, M. Bolotin-Fukuhara

Institut de Génétique et Microbiologie (UMR 8621) . Université Paris-Sud 91405 Orsay Cedex

In S.cerevisiae, the HAP heteromeric transcriptional complex is involved in the fermentation-respiration shift. This complex is composed of 4 subunits. Subunits 2, 3, and 5 (Hap2p, Hap3p, and Hap5p) are necessary for DNA binding, whereas the Hap4p subunit is glucose-repressed and contains a transcriptional activation domain. Hap4p is really the key regulator of the complex activity in response to carbon sources in S.cerevisiae.

While Hap2p, Hap3p, and Hap5p are conserved through evolution, the Hap4 subunit was postulated to be specific to S.cerevisiae and was not identified by sequence similarities in available databases. S. cerevisiae is a fermentative yeast which requires a complete reprogrammation of its transcriptional activity during the diauxic shift. In such cases, other yeasts which are mostly respiratory such as Kluyveromyces lactis, might indeed not require a HAP4 ortholog.

Our laboratoy has recently isolated by functional complementation the K.lactis HAP4 ortholog and shown that the deletion of the K.lactis gene has no detectable phenotype for the cell and does not regulate the transcriptional activity of several genes involved in mitochondrial functions. The question of the function of the regulatory network controled by the HAP complex is this yeast is therefore opened.

While the overall conservation of the two Hap4p proteins is poor, sequence comparison revealed the presence of two short (16 and 11 aminoacids respectively) conserved regions which we used to search for HAP4 orthologs in the Genolevure database where partial random sequencing of 13 different hemiascomyceteous yeasts are conserved. In that way, six pretenders have been found in Zygosaccharomyces rouxii, Saccharomyces servazii, Saccharomyces exigus, Pichia angusta and Pichia sorbitophila. The last one seems to contain at least two pretenders whereas only one copy was detected in other yeasts. We also identified a partial sequence for K.lactis which turned out to be identical to the sequence of KlHAP4 and validated our search.

Tools are now available to study the role of the Hap4p protein in these metabolically distinct yeast species. These studies might also bring some clues about the evolution of these yeasts from one metabolism to another through the study of the regulatory networks controled by the Hap4p orthologs.