Histone H1 Expressed in Saccharomyces cerevisiae Binds to Chromatin and Affects Survival, Growth, Transcription, and Plasmid Stability but Does Not Change Nucleosomal Spacing

Abstract

Histone H1 is proposed to serve a structural role in nucleosomes and chromatin fibers, to affect the spacing of nucleosomes, and to act as a general repressor of transcription. To test these hypotheses, a gene coding for a sea urchin histone H1 was expressed from the inducible GAL1 promoter in Saccharomyces cerevisiae by use of a YEp vector for high expression levels (strain YCL7) and a centromere vector for low expression levels (strain YCL1). The H1 protein was identified by its inducibility in galactose, its apparent molecular weight, and its solubility in 5% perchloric acid. When YCL7 was shifted from glucose to galactose for more than 40 h to achieve maximal levels of H1, H1 could be copurified in approximately stoichiometric amounts with core histones of Nonidet P-40-washed nuclei and with soluble chromatin fractionated on sucrose gradients. While S. cerevisiae tolerated the expression of low levels of H1 in YCL1 without an obvious phenotype, the expression of high levels of H1 correlated with greatly reduced survival, inhibition of growth, and increased plasmid loss but no obvious change in the nucleosomal repeat length. After an initial induction, RNA levels for GAL1 and H1 were drastically reduced, suggesting that H1 acts by the repression of galactose-induced genes. Similar effects, but to a lower extent, were observed when the C-terminal tail of H1 was expressed.