An improved MS2 system for accurate reporting of the mRNA life cycle

An improved MS2-tagging system for live-cell RNA imaging allows faithful monitoring of the mRNA life cycle, overcoming degradation artifacts associated with previous versions and having implications regarding mRNA regulation in yeast. The MS2–MCP system enables researchers to image multiple steps of the mRNA life cycle with high temporal and spatial resolution. However, for short-lived mRNAs, the tight binding of the MS2 coat protein (MCP) to the MS2 binding sites (MBS) protects the RNA from being efficiently degraded, and this confounds the study of mRNA regulation. Here, we describe a reporter system (MBSV6) with reduced affinity for the MCP, which allows mRNA degradation while preserving single-molecule detection determined by single-molecule FISH (smFISH) or live imaging. Constitutive mRNAs (MDN1 and DOA1) and highly-regulated mRNAs (GAL1 and ASH1) endogenously tagged with MBSV6 in Saccharomyces cerevisiae degrade normally. As a result, short-lived mRNAs were imaged throughout their complete life cycle. The MBSV6 reporter revealed that, in contrast to previous findings, coordinated recruitment of mRNAs at specialized structures such as P-bodies during stress did not occur, and mRNA degradation was heterogeneously distributed in the cytoplasm.