Changes in the levels of three different classes of histone mRNA during murine erythroleukemia cell differentiation.

Abstract

We used a gene-specific Si nuclease assay to study the changes in steady-state mRNA levels of several core histone variants during the differentiation of murine erythroleukemia cells. These studies allowed us to distinguish three distinct expression classes of histone genes. The expression of the major replication-dependent class of histone genes was tightly linked to DNA synthesis. The concentrations of these transcripts decreased rapidly as cell division slowed during the process of differentiation. In contrast, the replication-independent H3.3 transcript levels were constitutively maintained throughout differentiation and were unaffected by inhibitors of DNA or protein synthesis. We also identified among the cloned histone genes used as probes a third expression class, the partially replication-dependent variants. Expression of these transcripts became transiently uncoupled from the reduced rate of DNA synthesis accompanying the early stages of differentiation. We show that their synthesis is sensitive to the DNA synthesis inhibitor hydroxyurea but that selective uncoupling from DNA synthesis of these histone mRNAs occurs at a specific stage of differentiation. We present several hypotheses to explain how this might be accomplished. The expression characteristics of the mRNAs studied coincided with those of the proteins for which they code, indicating that changes in the relative levels of the different variants is mediated at least in part by changes in mRNA levels.Histones are a complex family of proteins responsible for organizing the nucleosomal structure of eucaryotic chromatin (22,27,32). In mammals, each histone subtype, with the possible exception of H4, is represented by several nonallelic primary sequence variants which display distinct patterns of expression during differentiation (5,6,14,20,24,(49)(50)(51)(52)(53)(54). Nuclease digestion studies indicate that these variants are nonrandomly distributed between actively transcribed and inactive DNA sequences, suggesting that the histones may play a role in establishing or at least maintaining different chromatin conformations (35,47,48).Histone variants are classified according to their expression characteristics (53, 54). The major class, the replicationdependent variants, predominate in rapidly dividing tissues, and their synthesis is tightly linked to DNA synthesis (7,9,16,34,49). The mRNAs for these variants begin to accumulate near the G1-S border of the cell cycle, reach maximum levels at mid-S phase, and then rapidly decline as DNA synthesis is completed (1,2,8,11,21,33). Regulation of replication-dependent histone mRNAs occurs at the level of transcription and stability (21, 40).Considerably less is known about the regulation of expression of the other classes of histones. In studies of regenerating liver, partially replication-dependent histones have been identified as a group of variants which are induced with the onset of DNA synthesis but not completely repressed after cessation of DNA synthesis (53, 54). The expression of another class ...

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An alternative pathway of histone mRNA 3′ end formation in mouse round spermatids

“…This gene, which encodes a H2a.2 protein (20), has been described as a 'partially' replication-dependent gene (21). This class of genes are induced with the onset of DNA synthesis, similar to the replication-dependent histone genes, but are not completely repressed after DNA synthesis ends.…”

Epigenetic and replacement roles of histone variant H3.3 in reproduction and development

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