Subcellular Localization of a Bacterial Regulatory RNA

Eukaryotes and bacteria regulate the activity of some proteins by localizing them to discrete subcellular structures, and eukaryotes localize some RNAs for the same purpose. To explore whether bacteria also spatially regulate RNAs, the localization of tmRNA was determined using fluorescence in situ hybridization. tmRNA is a small regulatory RNA that is ubiquitous in bacteria and that interacts with translating ribosomes in a reaction known as transtranslation. In Caulobacter crescentus, tmRNA was localized in a cell-cycle-dependent manner. In G₁-phase cells, tmRNA was found in regularly spaced foci indicative of a helix-like structure. After initiation of DNA replication, most of the tmRNA was degraded, and the remaining molecules were spread throughout the cytoplasm. Immunofluorescence assays showed that SmpB, a protein that binds tightly to tmRNA, was colocalized with tmRNA in the helix-like pattern. RNase R, the nuclease that degrades tmRNA, was localized in a helix-like pattern that was separate from the SmpBtmRNA complex. These results suggest a model in which tmRNASmpB is localized to sequester tmRNA from RNase R, and localization might also regulate tmRNA-SmpB interactions with ribosomes.