Genomic evolution of the proteasome system among Hemiascomycetous yeasts

Components of the proteasome-ubiquitin pathway are highly conserved throughout eukaryotic organisms. In S. cerevisiae, the expression of proteasomal genes is subject to concerted control by a transcriptional regulator, Rpn4p, interacting with a highly conserved cis-regulatory element, PACE, located in the upstream regions of these genes. Taking advantage of sequence data accumulated from 15 Hemiascomycetes, we performed an in silico study to address the problem of how this system might have evolved among these species. We found that in all these species the Rpn4p homologues are well conserved in terms of sequence and characteristic domain features. The PACE patterns turned out to be nearly identical among the Saccharomyces sensu stricto species, whereas in the evolutionary more distant species the putatively functional cis-regulatory motifs revealed deviations from the canonical PACE nonamere sequence in one or two nucleotides. Our findings suggest that during evolution of the Hemiascomycetes such slightly divergent ancestral motifs have converged into a unique PACE element for the majority of the proteasomal genes within the most recent species of this class. Likewise, the Rpn4 factors within the most recent species of this class show a higher degree of similarity in sequence than their ancestral counterparts. By contrast, we did not detect PACE-like motifs among the proteasomal genes in other eukaryotes, such as S. pombe, several filamentous fungi, A. thaliana, or humans, leaving the interesting question which type of concerted regulation of the proteasome system has developed in species other than the Hemiascomycetes.