AN UNUSUAL HISTONE H3 SPECIFIC FOR EARLY MACRONUCLEAR DEVELOPMENT IN EUPLOTES-CRASSUS

Characterization of the histone H3 genes of the ciliated protozoan Eup lotes crassus indicates that one gene functions only during the sexual phase of the life cycle. Maximum expression of this gene, as judged b y transcript accumulation, correlates with DNA replications leading to polytenization of the micronuclear chromosomes before massive DNA eli mination, which produces a transcriptionally active macronucleus. Tran scripts of the other gene accumulate primarily during vegetative growt h and in the sexual phase of the life cycle during replication phases not related to polytenization. Although both histone H3 genes encode p roteins that are fairly divergent in sequence at the amino terminus, t he meiotic/polytene-specific histone H3 contains two insertions in the amino terminus that increase the size of the protein by 15 amino acid s, Analysis of micrococcal nuclease digests of chromatin using hybridi zation probes specific for micronuclear vs. macronuclear sequences ind icates that a change in nucleosomal spacing correlates with the maxima l expression of the meiotic/polytene-specific histone H3 gene. Thus, w e surmise that this unusual histone H3 mag play a key role in targetin g DNA sequences for either transcriptional activation and retention in the macronucleus or heterochromatization and elimination.