In bacteria, the 5' mRNA coding region plays an important role in determining translation output. Here, we report synthetic sequences that when placed in the 5'-mRNA coding region, leading to recombinant proteins containing short N-terminal extensions, virtually abolish, enhance or produce intermediate expression levels of green fluorescent protein in Escherichia coli. At least in one case, no apparent effect on protein stability was observed, pointing to RNA level effects as the principal reason for the observed expression differences. Targeting a synonymous codon library to the 5' coding sequence allowed tuning of protein expression over ~300-fold with preservation of amino acid identity. This approach is simple and should be generally applicable in bacteria. The data support that features in the 5' mRNA coding region near the AUG start codon are key in determining translation output and hence is important to recombinant and, most certainly, endogenous gene expression.