Myb Genes in Ciliates: a Common Origin with the Myb Protooncogene?

In animals, the protooncogene myb family is characterized by a DNA-binding domain (so-called MYB domain), which consists of 3 imperfect tandem repeats of a helix-turn-helix motif. Homologous genes have been characterized in plants and also in Dictyostelium discoideum. However, in plants, the myb family is more diverse and displays 2 types of MYB domains: the animal-like 3 repeats (MYB-3R) and the 2 repeats (MYB-2R) domains. The question is therefore raised as to the putative existence of genes with MYB-3R and/or MYB-2R domains in their last common unicellular ancestor. Here, we present evidence that in ciliates like in plants, both types of domain exist. A gene having a MYB-3R domain has been identified in the oxytrichid Sterkiella histriomuscorum and a gene having a MYB-2R domain has been identified in the euplotid Euplotes aediculatus. Both genes are expressed during the vegetative growth of the cells. A conserved intron exists in the gene of Sterkiella and phylogenetical analyses show that the 2 ciliate genes belong to the myb protooncogene family as deeply split lineages. This is the first report of a myb homolog in a ciliated protist, thus, confirming its origin in strict unicellular eukaryotes.