Low degree of ubiquitination of histone 2A in the dipteran Chironomus tentans

A conserved sequence in histone H2A which is a ubiquitination site in higher eucaryotes is not required for growth in Saccharomyces cerevisiae

Histones H2A and H2B are modified by ubiquitination of specific lysine residues in higher and lower eucaryotes. To identify functions of ubiquitinated histone H2A, we studied an organism in which genetic analysis of histones is feasible, the yeast Saccharomyces cerevisiae. Surprisingly, immunoblotting experiments using both anti-ubiquitin and anti-H2A antibodies gave no evidence that S. cerevisiae contains ubiquitinated histone H2A. The immunoblot detected a variety of other ubiquitinated species. A sequence of five residues in S. cerevisiae histone H2A that is identical to the site of H2A ubiquitination in higher eucaryotes was mutated to substitute arginines for lysines. Any ubiquitination at this site would be prevented by these mutations. Yeast organisms carrying this mutation were indistinguishable from the wild type under a variety of conditions. Thus, despite the existence in S. cerevisiae of several gene products, such as RAD6 and CDC34, which are capable of ubiquitinating histone H2A in vitro, ubiquitinated histone H2A is either scarce in or absent from S. cerevisiae. Furthermore, the histone H2A sequence which serves as a ubiquitination site in higher eucaryotes is not essential for yeast growth, sporulation, or resistance to either heat stress or UV radiation.

A conserved sequence in histone H2A which is a ubiquitination site in higher eucaryotes is not required for growth in Saccharomyces cerevisiae

Histones H2A and H2B are modified by ubiquitination of specific lysine residues in higher and lower eucaryotes. To identify functions of ubiquitinated histone H2A, we studied an organism in which genetic analysis of histones is feasible, the yeast Saccharomyces cerevisiae. Surprisingly, immunoblotting experiments using both anti-ubiquitin and anti-H2A antibodies gave no evidence that S. cerevisiae contains ubiquitinated histone H2A. The immunoblot detected a variety of other ubiquitinated species. A sequence of five residues in S. cerevisiae histone H2A that is identical to the site of H2A ubiquitination in higher eucaryotes was mutated to substitute arginines for lysines. Any ubiquitination at this site would be prevented by these mutations. Yeast organisms carrying this mutation were indistinguishable from the wild type under a variety of conditions. Thus, despite the existence in S. cerevisiae of several gene products, such as RAD6 and CDC34, which are capable of ubiquitinating histone H2A in vitro, ubiquitinated histone H2A is either scarce in or absent from S. cerevisiae. Furthermore, the histone H2A sequence which serves as a ubiquitination site in higher eucaryotes is not essential for yeast growth, sporulation, or resistance to either heat stress or UV radiation.

Presence of histone H1 on an active Balbiani ring gene

We have investigated whether histone H1 is present on active Balbiani ring genes in the salivary glands of Chironomus tentans using immunoelectron microscopy. The genes were studied in two activity states: maximally activated genes with a fully extended template and repressed genes in a 30 nm fiber conformation. Histone H1 was recorded on the gene in both conformations; the immunosignal was considerably stronger in the transcriptionally active state, probably reflecting the increased accessibility of histone H1 to the antibody in unfolded versus compacted chromatin. We conclude that during transcription the DNA template is extended and the nucleosomes are disrupted at the RNA polymerases, but histone H1, and most likely also the core histones, remains bound to the template.