Simultaneous Detection of mRNA and Protein in S. cerevisiae by Single-Molecule FISH and Immunofluorescence.

Single-molecule fluorescent in situ hybridization (smFISH) enables the detection and quantification of endogenous mRNAs within intact fixed cells. This method utilizes tens of singly labeled fluorescent DNA probes hybridized against the mRNA of interest, which can be detected by using standard wide-field fluorescence microscopy. This approach provides the means to generate absolute quantifications of gene expression within single cells, which can be used to link molecular fluctuations to phenotypes. To be able to correlate the expression of an mRNA and a protein of interest in individual cells, we combined smFISH with immunofluorescence (IF) in yeast cells. Here, we present our smFISH-IF protocol to visualize and quantify two cell cycle-controlled mRNAs (CLN2 and ASH1) and the cell cycle marker alpha-tubulin in S. cerevisiae. This protocol, which is performed over 2 days, can be used to visualize up to three colors at the time (i.e., two mRNAs, one protein). Even if the described protocol is designed for S. cerevisiae, we think that the considerations discussed here can be useful to develop and troubleshoot smFISH-IF protocols for other model organisms.