An unusual histone H3 specific for early macronuclear development in Euplotes crassus.

Characterization of the histone H3 genes of the ciliated protozoan Euplotes crassus indicates that one gene functions only during the sexual phase of the life cycle. Maximum expression of this gene, as judged by transcript accumulation, correlates with DNA replications leading to polytenization of the micronuclear chromosomes before massive DNA elimination, which produces a transcriptionally active macronucleus. Transcripts of the other gene accumulate primarily during vegetative growth and in the sexual phase of the life cycle during replication phases not related to polytenization. Although both histone H3 genes encode proteins that are fairly divergent in sequence at the amino terminus, the meiotic/polytene-specific histone H3 contains two insertions in the amino terminus that increase the size of the protein by 15 amino acids. Analysis of micrococcal nuclease digests of chromatin using hybridization probes specific for micronuclear vs. macronuclear sequences indicates that a change in nucleosomal spacing correlates with the maximal expression of the meiotic/polytene-specific histone H3 gene. Thus, we surmise that this unusual histone H3 may play a key role in targeting DNA sequences for either transcriptional activation and retention in the macronucleus or heterochromatization and elimination.