Cellular Volume is a Global Controller of MRNA Abundance

Abstract

The biochemical view of cellular function requires that the concentrations of cellular constituents remain the same for cells to function properly. However, cells in a population can vary widely in both volume and the numbers of these constituent molecules, implying that densities must fluctuate widely from cell to cell, and it remains an open question to understand how cells can function in the presence of this molecular “noise”. Thus, we measured mRNA density in single cells using RNA fluorescence in situ hybridization (RNA-FISH). We find that many different species of mRNA in a given cell line exhibit constant density across a 4- to 6-fold volume range, each with their own characteristic density. We further found that quiescent and senescent cells also maintained the same characteristic density, despite changes in mean mRNA number. Thus, for many genes, RNA abundance is not random but precisely controlled to produce the appropriate amount of mRNA given the size of the cell, as though genes know how big the cell is. There must therefore exist some density conservation mechanism. We hypothesize that mRNA density is maintained due to increased transcription in larger cells. using RNA-FISH to visualize transcription at the single-gene level, we observe that overall transcriptional activity indeed scales with volume. We present a model explaining which transcriptional parameters govern density conservation despite transcription occurring in random bursts. Our findings suggest that global properties of RNA dynamics require a reassessment of our understanding of cellular heterogeneity and stochastic gene expression, and further suggest that evolution selects for particular genes to have certain transcriptional parameters in order to maintain their density. Our results suggest that density conservation is a natural consequence of the global feedback between cell volume and mRNA abundance independent of any specific mechanism.