Differential expression of linker histone variants in Euplotes crassus

Two genes have been cloned from the ciliate Euplotes crassus that encode proteins with sequence similarity to the linker histones from a variety of organisms. One gene, H1-1, is present on a 1.3-kb macronuclear DNA molecule and encodes a 16.2- kDa protein. The second gene, H1-2, is present on a 0.7-kb DNA molecule and encodes an 18.8-kDa protein. Both H1-1 and H1-2 are expressed in vegetative cells, but the two genes exhibit very different patterns of expression during macronuclear development. H1-1 transcripts accumulate during conjugation and during the final rounds of DNA amplification. H1-2 transcripts accumulate after the onset of polytene chromosome formation and remain high throughout the remainder of macronuclear development. H1-1 is the major perchloric-acid-soluble protein from macronuclei. The pattern of gene expression and the macronuclear location of the H1-1 protein indicate that H1-1 is the predominant linker histone in vegetative macronuclei.