Abstract
The sequences of the 5' untranslated regions (5'-UTRs) of mRNA alter gene expression across domains of life. Transcriptional modulators can be easily assayed through transcription termination, but translational regulators often require indirect, laborious methods. We have leveraged RelE's ribosome-dependent endonuclease activity to develop a quantitative assay to monitor translation initiation of cis-regulatory mRNAs. RelE cleavage accurately reports ligand-dependent changes in ribosome association for two translational riboswitches and provides quantitative information about each switch's sensitivity and range of response. RelE accurately reads out sequence-driven changes in riboswitch specificity and function and is quantitatively dependent upon ligand concentration. RelE cleavage similarly captures differences in translation initiation between yeast 5'-UTR isoforms. RelE cleavage can thus reveal a plethora of information about translation initiation in different domains of life.