A transgenic mouse for in vivo detection of endogenous labeled mRNA

live-cell single mrn A imaging is a powerful tool but has been restricted in higher eukaryotes to artificial cell lines and reporter genes. We describe an approach that enables livecell imaging of single endogenous labeled m rn A molecules transcribed in primary mammalian cells and tissue. We generated a knock-in mouse line with an ms 2 binding site (mBs ) cassette targeted to the 3 ′ untranslated region of the essential -actin gene. As -actin– mBs was ubiquitously expressed, we could uniquely address endogenous m rn A regulation in any tissue or cell type. We simultaneously followed transcription from the -actin alleles in real time and observed transcriptional bursting in response to serum stimulation with precise temporal resolution. We tracked single endogenous labeled m rn A particles being transported in primary hippocampal neurons. t he mBs cassette also enabled high-sensitivity fluorescence in situ hybridization (F ish ), allowing detection and localization of single -actin m rn A molecules in various mouse tissues.