An H1 histone gene-specific 5' element and evolution of H1 and H5 genes.

In previous studies we have shown that the H5 gene is not closely linked to the dispersed clusters of core and H1 histone genes. Here we emphasise features of H1 and H5 genes relevant to their expression in the chicken genome. Of particular note is an H1 gene-specific 5' element, 5' AAACACA 3' found upstream of all H1 genes studied to date. This "H1-box" is not found in the related H5 gene, which is expressed only in erythroid cells. A second aspect relates to generation of histone mRNA 3' termini. The H5 gene is shown to contain a remnant of the dyad symmetry element (as well as other conserved sequences) associated with core and H1-histone gene transcript 3' processing. However, it appears as if H5 has evolved a different mechanism in which the mRNA terminus (which is polyadenylated) is displaced downstream from the dyad element. The two clear differences noted here have the potential to affect transcriptional (H1-box) and post-transcriptional (3' terminus processing) regulation of H1 and H5 gene expression.