Gene expression toxicity is an important biological phenomenon and a major bottleneck in biotechnology. Escherichia coli BL21(DE3) is the most popular choice for recombinant protein production, and various derivatives have been evolved or engineered to facilitate improved yield and tolerance to toxic genes. However, previous efforts to evolve BL21, such as the Walker strains C41 and C43, resulted only in decreased expression strength of the T7 system. This reveals little about the mechanisms at play and constitutes only marginal progress towards a generally higher producing cell factory. Here, we restrict the solution space for BL21(DE3) to evolve tolerance and isolate a mutant strain Evo21(DE3) with a truncation in the essential RNase E. This suggests that RNA stability plays a central role in gene expression toxicity. The evolved rne truncation is similar to a mutation previously engineered into the commercially available BL21Star(DE3), which challenges the existing assumption that this strain is unsuitable for expressing toxic proteins. We isolated another dominant mutation in a presumed substrate binding site of RNase E that improves protein production further when provided as an auxiliary plasmid. This makes it easy to improve other BL21 variants and points to RNases as prime targets for cell factory optimisation.