Fundamental to the process of mammalian development is the timed and coordinated regulation of gene expression. This requires transcription of a precise subset of the total complement of genes. It is clear that chromatin architecture plays a fundamental role in this process by either facilitating or restricting transcription factor binding [1]. How such specialized chromatin structures are established to regulate gene expression is poorly understood. All eukaryotic organisms contain specialized histone variants with distinctly different amino acid sequences that are even more conserved than the major core histones [2]. On the basis of their highly conserved sequence, histone variants have been assumed critical for the function of mammalian chromatin; however, a requirement for a histone variant has not been shown in mammalian cells. Mice with a deletion of H1 degrees have been generated by gene targeting in ES cells, but these mice show no phenotypic consequences, perhaps due to redundancy of function [3]. Here we show for the first time that a mammalian histone variant, H2A.Z, plays a critical role in early development, and we conclude that this histone variant plays a pivotal role in establishing the chromatin structures required for the complex patterns of gene expression essential for normal mammalian development.

One way in which a distinct chromosomal domain could be established to carry out a specialized function is by the localized incorporation of specific histone variants into nucleosomes. H2AZ, one such variant of the histone protein H2A, is required for the survival of Drosophila melanogaster, Tetrahymena thermophila and mice (R. Faast et al., in preparation). To search for the unique features of Drosophila H2AZ (His2AvD, also referred to as H2AvD) that are required for its essential function, we have performed amino-acid swap experiments in which residues unique to Drosophila His2AvD were replaced with equivalently positioned Drosophila H2A.1 residues. Mutated His2AvD genes encoding modified versions of this histone were transformed into Drosophila and tested for their ability to rescue null-mutant lethality. We show that the unique feature of His2AvD does not reside in its histone fold but in its carboxy-terminal domain. This C-terminal region maps to a short alpha-helix in H2A that is buried deep inside the nucleosome core.

An efficient expression system in Escherichia coli for several biologically active insulin-like growth factor-I (IGF-I) fusion peptide analogues is described.

An H1 gene‐specific element (H1‐box, 5′‐AAACACA‐3′) modulates S‐phase expression of the gene in vivo as judged by analysis of transcripts from histone genes transfected into HeLa cells. Deletion or base‐substitution of the element causes a 15‐ to 30‐fold decrease in steady‐state H1 mRNA levels in randomly growing cells and eliminates cell cycle control of transcription in synchronized cells. Mutations within the H1‐specific element which abolish S‐phase control of transcription also eliminate binding of a sequence‐specific nuclear factor capable of binding specifically to this region in vitro. Transfection of multiple copies of H1‐box elements into cells drastically decreases H1 mRNA levels, mimicking the effect observed when the motif is rendered non‐functional by deletion or substitution mutagenesis. In contrast, introduction of mutated H1 elements into cells has no detectable effect. Together, these results imply that an interaction between the H1‐box and a sequence‐specific trans‐acting factor modulates transcriptional control of H1 genes in vivo.

The DNA sequence of two chicken histone H2B genes has been determined. Both genes code for the same H2B subtype. Except for conserved "promoter" elements, the sequences 5' to the protein coding regions are completely divergent, indicating that the genes are distantly related and are not evolving in concert. This presents an ideal situation for sequence comparisons. We have discovered a 13 bp, H2B specific homology block, 5' CTCATTTGCATAC 3' located close to the "TATA box". This motif is conserved in all H2B gene leader regions so far sequenced. One of the H2B genes is closely linked, in a divergent arrangement, to an H2A gene, and sequence data suggests that the linked genes share promoter elements.