Basonuclin is associated with the ribosomal RNA genes on human keratinocyte mitotic chromosomes

Basonuclin is a zinc finger protein mainly expressed in keratinocytes of the basal layer of epidermis and the outer root sheath of hair follicles. It is also found in abundance in the germ cells of testis and ovary. In cultured keratinocytes, basonuclin is associated with chromatin in all phases of the cell cycle, including mitosis. By immunocytochemical methods, we demonstrate here that in mitosis basonuclin is associated with the short arms of the acrocentric chromosomes and with other loci on many metaphase chromosomes of human keratinocytes. Using the evolutionarily highly conserved N-terminal pair of zinc fingers in an electrophoresis mobility shift assay, we demonstrate that the DNA target sequences of basonuclin on the acrocentric chromosomes are likely to be within the promoter region of the 45S rRNA gene transcription unit. DNase I footprinting shows that basonuclin zinc fingers interact with the upstream control element of this promoter, which is necessary for the high level of transcription of the rRNA genes. This result suggests that basonuclin may be a tissue-specific transcription factor for the ribosomal RNA genes.