Gene expression in living cells is dynamic and unstable, and fluctuations in transcription may be subject to stochastic regulation of processes including transcription factor and polymerase recruitment, and chromatin remodelling. One gene that has been shown to display dynamically variable transcription and marked heterogeneity between cells is prolactin (PRL). Time-lapse imaging of PRL-reporter gene expression in single rat pituitary GH3 cells revealed distinct long-term transcription cycles (~11 h) with a short on-phase (~4 h) and a longer off-phase including a refractory period (≥3 h). Regular transcriptional cycles were also observed in primary rat pituitary cells expressing a 160 kb PRL bacterial artificial chromosome containing the firefly luciferase gene.
A systems biology approach was implemented to enable the direct comparison of two different reporter genes (luciferase and d2EGFP) driven by identical PRL promoters within the same single pituitary cell over time. Mathematical reconstruction (MCMC) of transcription rates showed that transcription cycles from the two PRL promoters were out-of-phase, and hence there was no significant co-ordination between the behaviour of two loci in the same cell. Synchronisation between the timing of expression of the two promoters in the same cell was induced by treating the cells with the histone deacetylase inhibitor, trichostatin A, suggesting that histone acetylation has a key role in the co-ordination of the temporal kinetics of PRL transcription.
Our data indicate that cycles of PRL transcription are longer than the transcription cycles previously observed in other systems, and are largely asynchronous between cells as well as between identical transcription units in the same cell. If such stochastic and cyclical patterns of gene expression occur in living cells in the context of intact tissue, this might explain how tissues mount widely differing acute or chronic responses to environmental cues while maintaining a controlled average level of gene expression.