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Codon optimality is the primary contributor to the exceptional mutational sensitivity of CcdA antitoxin in its operonic context

Deep mutational scanning studies suggest that single synonymous mutations are typically silent and that most exposed, non active-site residues are tolerant to mutations. Here we show that the ccdA antitoxin component of the E.coli ccdAB toxin-antitoxin operonic system is unusually sensitive to mutations when studied in the operonic context. A large fraction (∼80%) of single codon mutations, including many synonymous mutations in the ccdA gene show inactive phenotypes that are correlated with the E.coli codon usage frequency but retain native-like binding affinity towards cognate toxin, CcdB. Therefore, the observed phenotypic effects are largely not due to alterations in protein structure or stability, consistent with the fact that a large region of CcdA is intrinsically disordered. In select cases, proteomics studies reveal altered ratios of CcdA:CcdB protein levels in vivo, suggesting that these mutations likely alter relative translation efficiencies of the two genes in the operon. We extend these results by predicting and validating single synonymous mutations that lead to loss of function phenotypes in the relBE operon upon introduction of rarer codons. Thus, in their native context, genes are likely to be more sensitive to both synonymous and non-synonymous point mutations than inferred from previous saturation mutagenesis studies.