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

We used a gene-specific S1 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.