Real-time dynamics of individual chemoreceptor mRNA molecules reveals translation hotspots at the inner membrane of Escherichia coli

Since bacteria lack a nucleus, the location of mRNA molecules is determined by the different characteristics of the encoded proteins, and the transcriptome is spatially arranged into cytosolic and membrane-associated mRNA. While translation of membrane protein-encoding mRNA has been studied in great mechanistic detail using biochemical methods, the spatiotemporal dynamics of this process remains poorly understood at the subcellular level. Here, we investigate the dynamics of individual fluorescently labelled mRNA molecules encoding the transmembrane serine chemoreceptor Tsr, to probe the mechanism of membrane protein translation. Analysis of tsr mRNA diffusion in the proximity of the plasma membrane revealed distinct diffusive modes that reflect the state of the mRNA molecule and its involvement in the process of active translation into the Sec secretion system. We find that the composition, and hence the fluidity of the membrane affects diffusion of membrane targeted mRNAs. Moreover, Tsr translation occurs in localized membrane regions, similar to eukaryotic hotspots. The hotspot localization coincides with the physical location of the transcribed gene, which itself is displaced towards the inner membrane. These findings suggest that inner membrane protein translation is a spatially defined process that occurs in hotspots, indicative of long-lived transertion sites. Our results show an additional layer of spatio-temporal structuring within the bacterial cell, thus revealing a qualitatively different understanding of the basic process of transcription and translation in bacteria. Significance Statement A large fraction of the bacterial proteome is directly synthesized into the inner membrane, and this process shapes the overall distribution of mRNA transcripts within the cell. Although highly dynamic in their nature, bacterial transcriptomes have mostly been studied in fixed cells. Here, we track individual mRNA molecules encoding the serine chemoreceptor in living bacterial cells and find that translation occurs in membrane hotspots that were previously exclusive to eukaryotes. Our results indicate an additional layer of spatio-temporal structuring within the bacterial cell that impacts our understanding of transcription and translation in bacteria.