Both HMG boxes in Hmo1 are essential for DNA binding in vitro and in vivo

Hmo1, a member of the high mobility group B family proteins in Saccharomyces cerevisiae, associates with the promoters of ribosomal protein genes (RPGs) to direct accurate transcriptional initiation. Here, to identify factors involved in the binding of Hmo1 to its targets and the mechanism of Hmo1-dependent transcriptional initiation, we developed a novel reporter system using the promoter of the RPG RPS5. A genetic screen did not identify any factors that influence Hmo1 binding, but did identify a number of mutations in Hmo1 that impair its DNA binding activity in vivo and in vitro. These results suggest that Hmo1 binds to its target promoters autonomously without any aid of additional factors. Furthermore, characterization of Hmo1 mutants showed that the box A domain plays a pivotal role in DNA binding and may be required for the recognition of structural properties of target promoters that occur in native chromatin.