| mRNAs play a central role in the transfer of genetic information; yet little is known about the location of this molecule in bacterial cells even though it determines where processes of translation and mRNA degradation can take place in the cell. Transcripts are assumed to freely diffuse and previous studies have reported apparent diffusion coefficients for bacterial mRNAs that predict that mRNA molecules will have enough time to diffuse and sample the entire cell compartment before they get degraded. However, the available methods are not well suited for the study of chromosomally-encoded mRNAs, as the conditions of synthesis and decay of mRNAs were not physiological.;I developed a sensitive Locked Nucleic Acid (LNA) Fluorescence In Situ Hybridization method (FISH) method to visualize and quantify the spatial distribution of multiple chromosomally-encoded mRNAs under conditions in which synthesis and degradation occur normally. I examined the localization of mRNAs in C. crescentus and E. coli with different characteristics. I found that even long lived, abundant or processed mRNAs remain localized as diffraction-limited spots to specific sub-cellular positions that correspond to their sites of synthesis. These results suggest that bacterial mRNAs display a limited dispersion from their sites of transcription, and therefore largely remain localized near their site of birth for their entire lifetime. Consistent with this idea, I showed that the limited dispersion of the mRNA restricts the mobility of ribosomes engaged in translation. The observation of mRNA localization in two distantly related bacterial species suggest that the localization of the mRNA is a general property of bacterial cells that allows them to spatially regulate the flow of genetic information.;In addition to translation, mRNA decay is an essential process in the regulation of gene expression. The stability of mRNA determines the number of times an mRNA is translated, and therefore, the amount of protein produced. I further examine the spatial organization of this process in living C. crescentus cells, I found that RNase E, the main enzyme in mRNA decay associates directly or indirectly to the DNA independently to its mRNA substrate. My findings have important implications on how genes are organized in the chromosome. The localization of the mRNA, and therefore, the synthesis of each protein near the gene locus, could explain the phenomenon of gene clustering in bacterial chromosomes. Together, my findings suggest that bacteria are able to spatially organize the processes of translation and mRNA decay by using the organization of the chromosome as their template instead of compartmentalizing them in membrane-bound organelles. The spatial organization of these three processes ensures a rapid and efficient flow of genetic information in bacteria. |
