Transcription factor substitution during the evolution of fungal ribosome regulation

Coordinated ribosomal protein (RP) gene expression is crucial for cellular viability, but the transcriptional network controlling this regulon has only been well characterized in the yeast Saccharomyces cerevisiae. We have used whole-genome transcriptional and location profiling to establish that, in Candida albicans, the RIP regulon is controlled by the Myb domain protein Tbf1 working in conjunction with Cbf1. These two factors bind both the promoters of RIP genes and the rDNA locus; Tbf1 activates transcription at these loci and is essential. Orthologs of Tbf1 bind TTAGGG telomeric repeats in most eukaryotes, and TTAGGG cis-elements are present upstream of RIP genes in plants and fungi, suggesting that Tbf1 was involved in both functions in ancestral eukaryotes. In all Hemiascomycetes, Rap1 substituted Tbf1 at telomeres, and, in the S. cerevisiae lineage, this substitution also occurred independently at RIP genes, illustrating the extreme adaptability and flexibility of transcriptional regulatory networks.