Gene activation at a distance and telomeric silencing are not affected by yeast histone H1

Abstract

Until recently, it was believed that the budding yeast Saccharomyces cerevisiae has no histone H1 gene. However, a search of the yeast genome database revealed a possible H1 homologue of 258 amino acids, termed yeast histone H1 (HHO1). The protein shows 36% identity to the human H1 core domain over a stretch of 93 amino acids. Unlike other H1 proteins, Hho1p has a second possible core domain which shows 43% identity to the first core domain. Since vertebrate H1 histone had been implied in gene repression as well as gene activation at a distance, we tested the effect of deleting the yeast H1-like gene on remote activation of a modified GAL1 promoter, which contains a synthetic GAL4 binding site close to the TATA box, and the natural UASG, consisting of four GAL4 binding sites. Different spacing up to 1.8 kb between the proximal binding site and the distal UASG enhancer revealed no differences in gene activation between wild-type and knockout strains. Overexpression of a heterologous histone H1 from sea urchin showed an overall inhibition of gene activation by the GAL1 promoter, whereas overexpression of the yeast histone H1 had no effect. Also, the expression of A1, ALPHA2 or SUC2 genes, all of which are known to be responsive to an altered chromatin structure, was unchanged in HHO1 knockout or HHO1-overexpressing strains when compared to wild-type cells. We also considered the possibility that HHO1 was involved in forming the heterochromatin at telomeres. On testing for telomeric silencing of a URA reporter gene introduced 1.3 kb away from the chromosomal end, we again observed no differences between wild-type and knockout strains. Thus, the yeast histone H1-like gene appears to have no role in gene activation at a distance or in silencing under the conditions tested. It remains to be seen whether the yeast H1 histone is a gene-specific regulator rather than a general chromatin-associated protein.

Histone H1 of Saccharomyces cerevisiae Inhibits Transcriptional Silencing

“…These results contrast with earlier reports that failed to detect any effect of Hho1p on silencing (Escher and Schaffner 1997;Patterton et al 1998). We believe there are two reasons for these discrepancies.…”

“…The deletion did not affect TRP1 or URA3 expression. This result is consistent with earlier findings that failed to detect an effect of HHO1 deletion on the silencing of subtelomeric reporter genes (Escher and Schaffner 1997;Patterton et al 1998). From this set of experiments we conclude that HHO1 does not act directly on the reporter genes, nor does it affect telomeric silencing.…”

Saccharomyces cerevisiae Linker Histone Hho1p Functionally Interacts with Core Histone H4 and Negatively Regulates the Establishment of Transcriptionally Silent Chromatin

“…1, compare 2 with 1, 10 with 9, and 14 with 13 on FOA). This is consistent with previous studies that revealed no role of HHO1 in transcriptional silencing (14,18,21). However, it is formally possible that Hho1p plays a role in silencing that is redundant with or antagonistic to functions of other factors, such as histones, involved in the formation and/or maintenance of silent chromatin.…”

“…However, deletion of HHO1 seems to have no effect on silencing at HM loci, telomeres, and rDNA (14,18,21,23). On the other hand, we found that overexpression of HHO1 has an inhibitory effect on silencing (28).…”

“…Deletion of HHO1 has no noticeable effect on cell growth (14,18,21) and causes a reduction in the transcripts of only 27 of about 6000 genes by a factor of 2 or more, indicating that Hho1p positively regulates the expression of only a subset of genes (22). There is evidence suggesting that Hho1p is inhibitory to homologous recombination (20,23).…”