Co-transcriptional translation amplifies mRNA noise in Escherichia coli

The variability in mRNA expression among isogenic cells exposed to identical environments is inherent. This variability originates from the inherent stochasticity of all processes underlying transcription. Although transcription and translation can occur simultaneously on the same mRNA molecule in bacteria, it is not well understood whether and how co-transcriptional translation contributes to variability in mRNA expression. Here, we studied the contribution of co-transcriptional translation to mRNA noise in E. coli cells. Using a transcription system physically decoupled from translation, we investigated the effect of ribosome binding to mRNA transcripts on variability in mRNA expression. We found that the propagation of RNAP noise to the mRNA level was increased by ribosome binding, leading to larger variations in the mRNA levels. We further demonstrated that ribosome binding increased the transcription initiation rate, resulting in the promoter becoming susceptible to RNAP noise. Co-transcriptional translation amplified transcriptional noise and modulated transcriptional bursting kinetics in bacterial cells.