LINKER HISTONES ARE NOT ESSENTIAL AND AFFECT CHROMATIN CONDENSATION IN-VIVO

We have (separately) disrupted all of the expressed macronuclear copie s of the HHO gene encoding macronuclear histone H1 and of the micronuc lear linker histone (MLH) gene encoding the protein MicLH in Tetrahyme na thermophila. These disruptions are shown to eliminate completely th e expression of each protein. Strains without either linker histone gr ow at normal rates and reach near-normal cell densities, demonstrating that linker histones are not essential for cell survival. Histone H1 knockout (Delta H1) cells have enlarged DAPI-stained macronuclei and n ormal-sized micronuclei, while MicLH knockout (Delta MicLH) cells have enlarged micronuclei and normal-sized macronuclei. Delta MicLH cells undergo mitosis normally. However, the micronuclear mitotic chromosome structure is less condensed. These studies provide evidence that link er histones are nonessential and are involved in chromatin packaging a nd condensation in vivo.