Histone H1 Is Dispensable for Methylation-Associated Gene Silencing in Ascobolus immersusand Essential for Long Life Span

Abstract

A gene encoding a protein that shows sequence similarity with the histone H1 family only was cloned in Ascobolus immersus. The deduced peptide sequence presents the characteristic three-domain structure of metazoan linker histones, with a central globular region, an N-terminal tail, and a long positively charged C-terminal tail. By constructing an artificial duplication of this gene, namedH1, it was possible to methylate and silence it by the MIP (methylation induced premeiotically) process. This resulted in the complete loss of the Ascobolus H1 histone. Mutant strains lacking H1 displayed normal methylation-associated gene silencing, underwent MIP, and showed the same methylation-associated chromatin modifications as did wild-type strains. However, they displayed an increased accessibility of micrococcal nuclease to chromatin, whether DNA was methylated or not, and exhibited a hypermethylation of the methylated genome compartment. These features are taken to imply thatAscobolus H1 histone is a ubiquitous component of chromatin which plays no role in methylation-associated gene silencing. Mutant strains lacking histone H1 reproduced normally through sexual crosses and displayed normal early vegetative growth. However, between 6 and 13 days after germination, they abruptly and consistently stopped growing, indicating that Ascobolus H1 histone is necessary for long life span. This constitutes the first observation of a physiologically important phenotype associated with the loss of H1.

ACKNOWLEDGMENTS

We are grateful to Annie Grégoire for help with some experiments and Solange Dehan for preparing all media. We thank Claudio Scazzocchio and colleagues, who kindly provided the A. nidulans H1 gene and data prior to publication. We thank J. Desgrès and A. Costa, who kindly determined the 5-methylcytosine contents of Ascobolus strains by high-performance liquid chromatography analyses of deoxyribonucleosides obtained after enzymatic hydrolysis of DNA. We thank Geneviève Almouzni, Vincent Colot, and Allyson Holmes for critical reading of the manuscript and members of the laboratories for discussions.

J.L.B. was a recipient of a fellowship from the French Ministère des Affaires Etrangères followed by a fellowship from the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina. This work was supported by grants from the Association pour la Recherche sur le Cancer (contracts 6200 and 9554) and the European Union (contract BIO4-96-0253).