Either of the Major H2A Genes but Not an Evolutionarily Conserved H2A.F/Z Variant of Tetrahymena thermophila Can Function as the Sole H2A Gene in the Yeast Saccharomyces cerevisiae

Abstract

H2A.F/Z histones are conserved variants that diverged from major H2A proteins early in evolution, suggesting they perform an important function distinct from major H2A proteins. Antisera specific for hv1, the H2A.F/Z variant of the ciliated protozoan Tetrahymena thermophila, cross-react with proteins from Saccharomyces cerevisiae. However, no H2A.F/Z variant has been reported in this budding yeast species. We sought to distinguish among three explanations for these observations: (i) that S. cerevisiae has an undiscovered H2A.F/Z variant, (ii) that the major S. cerevisiae H2A proteins are functionally equivalent to H2A.F/Z variants, or (iii) that the conserved epitope is found on a non-H2A molecule. Repeated attempts to clone an S. cerevisiae hv1 homolog only resulted in the cloning of the known H2A genes yHTA1 and yHTA2. To test for functional relatedness, we attempted to rescue strains lacking the yeast H2A genes with either the Tetrahymena major H2A genes (tHTA1 or tHTA2) or the gene (tHTA3) encoding hv1. Although they differ considerably in sequence from the yeast H2A genes, the major Tetrahymena H2A genes can provide the essential functions of H2A in yeast cells, the first such case of trans-species complementation of histone function. The Tetrahymena H2A genes confer a cold-sensitive phenotype. Although expressed at high levels and transported to the nucleus, hv1 cannot replace yeast H2A proteins. Proteins from S. cerevisiae strains lacking yeast H2A genes fail to cross-react with anti-hv1 antibodies. These studies make it likely that S. cerevisiae differs from most other eukaryotes in that it does not have an H2A.F/Z homolog. A hypothesis is presented relating the absence of H2A.F/Z in S. cerevisiae to its function in other organisms.

Essential and Nonessential Histone H2A Variants in Tetrahymena thermophila

“…These differences could reflect neutral mutations that do not cause functional distinctions between the two. This hypothesis is supported by the fact that either of the T. thermophila major H2A genes can function as the sole H2A gene in yeasts and each confers a similar coldsensitive phenotype (22). However, sequence comparisons indicate that the H2A1 or H2A2 proteins of T. thermophila and T. pyriformis are virtually identical between the two species whereas H2A1 and H2A2 within each species retain the divergent C-terminal ends (24).…”

“…Because the yeast Saccharomyces cerevisiae is unusual among eukaryotes in that it does not seem to have a minor H2A variant (22), it was not possible to test this hypothesis by gene knockout experiments in that organism. Functional analysis of the H2A.F/Z variant in the fission yeast Schizosaccharomyces pombe by gene knockout indicated that the gene was not essential but did affect growth and chromosome segregation (7).…”

“…Clearly, the difference in H2A variants between these yeasts and D. melanogaster is not due to multicellularity, since T. thermophila is unicellular like the yeasts but the HTA3 gene encoding its H2A.F/Z variant is essential as in D. melanogaster. We have argued elsewhere (22) that at least one function of H2A.F/Z variants in most organisms involves interaction with the linker histone H1 to create a chromatin state that is competent for transcription. Since, despite intensive study, no linker histones have been found in yeasts, we propose that the differences between them, on the one hand, and drosophila and T. thermophila, on the other, reflect an essential function for H2A.F/Z variants that involves interaction with linker histones.…”

Identification of Proliferation-Induced Genes in Arabidopsis thaliana (Characterization of a New Member of the Highly Evolutionarily Conserved Histone H2A.F/Z Variant Subfamily)

“…With the notable exception of Saccharomyces cerevisiae (Liu et al, 1996), H2A.F/Z variants have been identified in a number of highly evolutionarily distant organisms. The reported characterization of the Arabidopsis cH2AvAt cDNA demonstrates that H2A.F/Z histone variants have been conserved in plants and are highly homologous to their animal and lower eukaryote counterparts, indicating that these proteins evolved under similar strong selection pressure.…”

“…It has been proposed that H2A.F/Z variants could modify chromatin structure and in some way control the transcriptional competence of chromatin in response to extracellular signals and during development (Stargell et al, 1993;Liu et al, 1996).…”