A Fluorescence in Situ Hybridization Method To Quantify mRNA Translation by Visualizing Ribosome–mRNA Interactions in Single Cells

[Image: see text] Single-molecule fluorescence in situ hybridization (smFISH) is a simple and widely used method to measure mRNA transcript abundance and localization in single cells. A comparable single-molecule in situ method to measure mRNA translation would enable a more complete understanding of gene regulation. Here we describe a fluorescence assay to detect ribosome interactions with mRNA (FLARIM). The method adapts smFISH to visualize and characterize translation of single molecules of mRNA in fixed cells. To visualize ribosome–mRNA interactions, we use pairs of oligonucleotide probes that bind separately to ribosomes (via rRNA) and to the mRNA of interest, and that produce strong fluorescence signals via the hybridization chain reaction (HCR) when the probes are in close proximity. FLARIM does not require genetic manipulation, is applicable to practically any endogenous mRNA transcript, and provides both spatial and temporal information. We demonstrate that FLARIM is sensitive to changes in ribosome association with mRNA upon inhibition of global translation with puromycin. We also show that FLARIM detects changes in ribosome association with an mRNA whose translation is upregulated in response to increased concentrations of iron.