Hmo1, a member of HMGB family proteins in Saccharomyces cerevisiae, binds to and regulates the transcription of genes encoding ribosomal RNA and ribosomal proteins. The functional motifs of Hmo1 include two HMG-like motifs, box A and box B, and a C-terminal tail. To elucidate the molecular roles of the HMG-like boxes in DNA binding in vivo, we analyzed the DNA-binding activity of various Hmo1 mutants using ChIP or reporter assays that enabled us to conveniently detect Hmo1 binding to the promoter of RPS5, a major target gene of Hmo1. Our mutational analyses showed that box B is a bona fide DNA-binding motif and that it also plays other important roles in cell growth. However, box A, especially its first α-helix, contributes to DNA binding of Hmo1 by inducing self-assembly of Hmo1. Intriguingly, box A mediated formation of oligomers of more than two proteins on DNA in vivo. Furthermore, duplication of the box B partially alleviates the requirement for box A. These findings suggest that the principal role of box A is to assemble multiple box B in the appropriate orientation, thereby stabilizing the binding of Hmo1 to DNA and nucleating specific chromosomal architecture on its target genes.

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Journal Article

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Kasahara K, Higashino A, Unzai S, Yoshikawa H, Kokubo T

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