a single cell's protein and mRNA copy number for any given gene are uncorrelated

Okay. So let me preface this by letting you all know how excited I was by this article (must log in to Science magazine to view full article), on a 1-10 scale. I think I'm circling around an 8.

Anyway, so I've always assumed higher mRNA levels are directly correlated with increased gene expression, and therefore higher levels of the protein coded by that gene. Right?

Welp. Apparently, according to researchers at Harvard and the University of Toronto, that's not necessarily true, due to differences in how long mRNA and proteins last in cells. The study utilized a YFP (yellow fluorescent protein) and single molecule FISH which allowed the researchers to quantitatively assess levels of gene expression and mRNA molecules expressed by the YFP-tagged genes. Being that they used gram-negative E. coli cells for the purpose of this study, they controlled for the differences in bacterial gene transcripts by "providing quantitative analyses of both abundance and noise in the proteome and transcriptome on a single-cell level", says Taniguchi, et al. As Sunney Xie comments, "this provides a cautionary note for people who want to do single cell mRNA-profiling".

The main finding of this study is that a single cell's protein and mRNA copy number for any given gene are uncorrelated. As the authors wrote, the study highlights the disconnect between proteome and transcriptome analyses of a single cell. It's also important in understanding how cells coordinate the expression of proteins that work together, such as multi-subunit proteins or proteins that serve within metabolic cycles.