Ribosome demand links transcriptional bursts to protein expression noise

The translation process modulates stochastic fluctuations in mRNA numbers and determines noise in protein expression. Studies across different organisms have revealed a positive correlation between translational efficiency and protein noise. However, the molecular basis of this correlation has remained unknown. Through stochastic modeling of translation in single mRNA molecules and empirical measurement of protein noise, we demonstrate that ribosome demand associated with high translational efficiency leads to the correlation between translational efficiency and protein noise. We also show that this correlation is present only in promoters with bursty transcription, where ribosome demand varies with stochastic fluctuations in mRNA numbers. Thus, our work shows how transcriptional bursts are translated into protein noise, which has implications for investigating noise and phenotypic heterogeneity across biological systems.

Methods
Clones of wild-type GFP gene and each mutant variant were grown in SCD medium (0.67% YNB + 0.079% complete synthetic supplement + 2% glucose) at 30℃ with shaking at 220 rpm. Yeast cells from glycerol stock were inoculated in SCD medium, were grown for 24 hrs, and were then diluted 1:100 in fresh medium. This process was repeated one more time.  The cells were then diluted 1:50 in fresh SCD and were grown for 4 hours. In the next step, cells were centrifuged and washed twice with 1X PBS (Phosphate Buffered Saline) buffer, and were finally resuspended in 1X PBS buffer for flow cytometry. In flow cytometry, the data were acquired for 2,00,000 cells for each sample in a BD LSR Fortessa cell analyzer.