mitotic cells maintain low levels of transcription

Mitotic chromosome condensation has been thought to interfere with chromatin-based processes, such as transcription. For this reason, it is thought that the genome is transcriptionally silenced during mitosis and that transcription is reactivated at mitotic exit. Zaret and colleagues now show that mitotic cells maintain low levels of transcription and that rates of transcription increase as cells progress through, and as they exit, mitosis.

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Previous studies indicated the presence of elongating RNA polymerase II in prometaphase-arrested cells, suggesting that transcription occurs in mitotic cells. To investigate whether mitotic cells are transcriptionally active, the authors used a cell-permeable uridine analogue, 5-ethynyluridine, which is incorporated into newly synthesized transcripts. This analogue was coupled to biotin, enabling easy pulldown using streptavidin-coated beads, followed by RNA sequencing. A considerable number of transcripts (8,074 transcripts, corresponding to 3,689 different genes) were detected in cells arrested in prometaphase following nocodazole treatment. Mitotic expression of selected transcripts was confirmed by PCR. Moreover, incorporation of fluorescent UTP was visualized on the arms of condensed chromosomes, indicating active RNA synthesis. Collectively, these data provided strong evidence that transcription persists in mitosis. In general, in mitotic cells, transcription was considerably reduced (fivefold on average) in comparison with asynchronously growing cells. However, levels of 789 mitotic transcripts (corresponding to 484 genes) were higher in prometaphase-arrested cells than in asynchronous cells; gene ontology assignment revealed that these transcripts were linked to the composition of the extracellular space and cell–matrix interactions, protein metabolism and transcription.

It has been previously shown that transcription is reactivated at mitotic exit, with a transcriptional burst occurring ∼90 min after release of cells from the prometaphase block. However, in the present study the levels of 927 transcripts were found to increase already 40 min after nocodazole washout. Gene ontology analysis revealed that these early-activated genes primarily function in intracellular organization (prominently in the formation of membrane compartments) and in translation. A second wave of transcription activation was observed between 80 and 105 min after release from mitotic arrest, and these genes related to cell adhesions. Genes activated in a third wave (at 165–300 min, around the time of entry into G1 phase) were linked to the cell cycle and DNA replication. Interestingly, the increase in cell type-specific transcripts was generally observed at later timepoints (towards mitotic exit), suggesting that genes regulating basic cell functions and cell growth are prioritized over genes with more specialized functions.

Overall, low-level transcription persists in mitosis and high-level transcription is gradually reactivated during progression through mitosis and into G1. The biological significance of mitotic transcription remains to be determined, although it is plausible that the maintenance of gene expression during mitosis supports daughter cell integration into the surrounding extracellular space following division and primes them for the robust gene expression necessary for their growth and basic functions.