The mouse histone H1 genes: gene organization and differential regulation.

There are six mouse histone H1 genes present in the histone gene cluster on mouse chromosome 13. These genes encode five histone H1 variants expressed in somatic cells, H1a to H1e, and the testis-specific H1t histone. Two of the genes that have not been assigned previously to the five somatic H1 subtypes have been identified as encoding the H1b and H1d subtypes. Three of the H1 genes, H1a, H1c and H1t, are present on an 80 kb segment of DNA that contains nine core histone genes. Two others, H1d and H1e, are present in a second patch, while the H1b gene is at least 500 kb away in a patch containing 14 core histone genes. The histone H1 genes are differentially expressed. All five genes for the somatic histone H1 proteins are expressed in exponentially growing cells. However, the levels of H1a, H1b and H1d mRNAs are greatly reduced in cells that are terminally differentiated or arrested in G0, while the H1c and H1e mRNAs continue to be expressed. In addition to the major RNA that ends at the stem-loop, the H1c gene expresses a longer, polyadenylated mRNA in differentiated cells, although in varying amounts. None of the other histone H1 genes encodes detectable amounts of polyadenylated mRNAs.

Mice develop normally without the H1(0) linker histone.

H1 histones bind to the linker DNA between nucleosome core particles and facilitate the folding of chromatin into a 30-nm fiber. Mice contain at least seven nonallelic subtypes of H1, including the somatic variants H1a through H1e, the testis-specific variant H1t, and the replacement linker histone H1(0). H1(0) accumulates in terminally differentiating cells from many lineages, at about the time when the cells cease dividing. To investigate the role of H1(0) in development, we have disrupted the single-copy H1(0) gene by homologous recombination in mouse embryonic stem cells. Mice homozygous for the mutation and completely lacking H1(0) mRNA and protein grew and reproduced normally and exhibited no anatomic or histologic abnormalities. Examination of tissues in which H1(0) is normally present at high levels also failed to reveal any abnormality in cell division patterns. Chromatin from H1(0)-deficient animals showed no significant change in the relative proportions of the other H1 subtypes or in the stoichiometry between linker histones and nucleosomes, suggesting that the other H1 histones can compensate for the deficiency in H1(0) by occupying sites that normally contain H1(0). Our results indicate that despite the unique properties and expression pattern of H1(0), its function is dispensable for normal mouse development.

Isolation and characterization of two replication-dependent mouse H1 histone genes.

Mice contain at least seven nonallelic forms of the H1 histones, including the somatic variants H1a-e and less closely related variants H1 degrees and H1t. The mouse H1 degrees and H1c (H1var.1) genes were isolated and characterized previously. We have now isolated, sequenced and studied the expression properties of two additional mouse H1 genes, termed H1var.2 and H1var.3. Extensive amino acid and nucleotide sequence comparisons were made between the two genes and other mammalian H1 histone genes. A high degree of nucleotide sequence identity was seen between the H1var.2, rat H1d and human H1b genes, even well beyond the coding region, indicating that these genes are likely homologues. Unlike the previously characterized mouse H1var.1 gene which produces both nonpolyadenylated and polyadenylated mRNAs, the H1var.2 and H1var.3 genes produce only typical, replication dependent, nonpolyadenylated mRNAs.