Regulation of eukaryotic gene expression occurs at multiple levels, and can vary depending on the gene. For example, the subcellular expression of β-actin is controlled by the localisation of its mRNA to actin-rich peripheral regions of migrating cells. On page , Yaron Shav-Tal and colleagues report a novel system for spatio-temporal quantification of β-actin expression. They create a human cell line expressing a transcriptionally inducible form of the chicken β-actin gene (ACTB) that enables visualisation of the gene, the transcribed mRNA and the translated protein in individual cells. They first show that the rtTA transcription factor binds to the ACTB promoter only transiently, for an average of 40 seconds. Four-dimensional imaging shows that, in this inducible system, the rate of transcription gradually increases to a maximum after ~1 hour, then gradually decreases. In agreement with previous findings, the authors also report that β-actin mRNA is elongated at a speed of 3.3 kb/minute and, once exported from the nucleus, moves through the cytoplasm by diffusion. Notably, β-actin mRNA that is stimulated to localise to the cell periphery derives from a pre-existing mRNA pool, rather than from newly transcribed mRNA produced following induction. So, the initial localization of β-actin mRNA to the cell periphery is not coupled to the initiation of β-actin transcription in the nucleus. This is the first study to follow the complete cellular pathway of a protein-coding mammalian mRNA in live cells.
