Cl. Jahn et al., AN UNUSUAL HISTONE H3 SPECIFIC FOR EARLY MACRONUCLEAR DEVELOPMENT IN EUPLOTES-CRASSUS, Proceedings of the National Academy of Sciences of the United Statesof America, 94(4), 1997, pp. 1332-1337
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.