Yeast may not contain histone H1: the only known 'histone H1-like' protein in Saccharomyces cerevisiae is a mitochondrial protein.

It is likely that histone H1 is involved in the condensation of chromatin in eukaryotes. However, both the presence of histone H1 in yeast and the extent of yeast chromatin condensation are controversial. A 20 kD protein copurifies with yeast chromatin and was shown by other investigators to have characteristics of histone H1 protein. In an attempt to obtain a positive identification of the 20 kD protein, we purified the protein to homogeneity and raised antibodies against it. We show here by immunofluorescence that the 20 kD protein does not localize to the nucleus but to cytoplasmic particles resembling mitochondria. Furthermore, we show by Western-blot analysis that anti-20 kD protein antibodies react to protein isolated from purified mitochondria. Finally, we present evidence based on size, charge, amino acid composition and immunological cross reactivity to suggest that the yeast 20 kD protein is likely to be the mitochondrial DNA-binding HM protein. This leaves no candidate for histone H1 in yeast.