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Vegetative cells of the ciliated protozoan Tetrahymena thermophila contain a transcriptionally active macronucleus and a transcriptionally inactive micronucleus. Although structurally and functionally dissimilar, these nuclei are products of a single postzygotic division during conjugation, the sexual phase of the life cycle. Immunocytochemical analyses during growth, starvation, and conjugation were used to examine the nuclear deposition of hv1, a histone H2A variant that is found in macronuclei and thought to play a role in transcriptionally active chromatin. Polyclonal antisera were generated using whole hv1 protein and synthetic peptides from the amino and carboxyl domains of hv1. The transcriptionally active macronuclei stained at all stages of the life cycle. Micronuclei did not stain during growth or starvation but stained with two of the sera during early stages of conjugation, preceding the stage when micronuclei become transcriptionally active. Immunoblot analyses of fractionated macro- and micronuclei confirmed the micronuclear acquisition of hv1 early in conjugation. hv1 staining disappeared from developing micronuclei late in conjugation. Interestingly, the carboxy-peptide antiserum stained micronuclei only briefly, late in development. The detection of the previously sequestered carboxyl terminus of hv1 may be related to the elimination of hv1 during the dynamic restructing of micronuclear chromatin that occurs as the micronucleus enters a transcriptionally incompetent state that is maintained during vegetative growth. These studies demonstrate that the transcriptional differences between macro- and micronuclei are associated with the loss of a chromatin component from developing micronuclei rather than its de novo appearance in developing macronuclei and argue that hv1 functions in establishing a transcriptionally competent state of chromatin.

genes and development

Temporal and spatial association of histone H2A variant hv1 with transcriptionally competent chromatin during nuclear development in Tetrahymena thermophila.