Isolation and characterization of two mouse H1 histone genes encoding differentiation-inducible polyA+mRNAs

Abstract

The regulation of H1 histone gene expression during terminal differentiation of mammalian cells was studied. The model system used for this work consists of murine erythroleukemia (MEL) cells, transformed erythroid precursors blocked at an early stage in the differentiation program. Treatment with a variety of chemical agents causes the cells to reinitiate a terminal differentiation program culminating in extensive hemoglobin synthesis.;I identified two H1 histone genes, H1(0) and H1.var.1, that were rapidly induced during the early period of MEL cell differentiation. Their cDNA and genomic clones were isolated and sequenced. The timing of their induction suggests that they may play an important role in the differentiation switch.;Interestingly, both H1 genes were found to encode two forms of mature mRNA produced by different 3{dollar}\sp\prime{dollar}-end processing. The H1(0) gene produced two polyadenylated mRNAs differing in the length of the 3{dollar}\sp\prime{dollar} untranslated region. The H1.var.1 gene generated both a short nonpolyA mRNA which is cell cycle regulated and a polyA mRNA which is not cell cycle controlled. H1.var.1 is the first example of a histone gene that produces two types of mRNA by using simultaneously both mechanisms of mRNA 3{dollar}\sp\prime{dollar}-end formation. The levels of the two H1.var.1 transcripts were independently regulated and under several physiological conditions showed inverse regulation. The production of a polyadenylylated mRNA from an otherwise cell cycle regulated histone gene may allow for continued synthesis of the histone protein when DNA synthesis ceases in nondividing cells.;The rapid induction of the two H1 genes following treatment with differentiation inducing agents correlated with a rapid decline in the expression of the cellular protooncogenes c-myc and c-myb. Previous studies have shown that transfection of MEL cells with c-myc or c-myb expression vectors that cause deregulated expression of the protooncogene also leads to a strong inhibition of MEL cell differentiation. The transfected cells were used to show that induction of the two H1 genes is specifically and negatively regulated by c-myc. These two H1 genes are among the first examples of cellular genes that are regulated by c-myc. Further studies with these genes may allow us to understand how c-myc regulates gene expression.