Fig. 3.

Sequence logos for essential, substrate-specific, and non-essential residue classes. a Essential residue class. For this class, the sequences obtained from the antibiotic-selected library are dominated by the wild-type residues and sometimes physicochemically similar residues. b Substrate-specific residue class. For this class, the distribution of residue types observed is dependent on the antibiotic used for selection. For example, a broad distribution of amino-acid types is observed among the sequences from ampicillin-selected libraries, whereas a narrower range of amino-acid types is observed among sequences from imipenem-selected libraries. c Non-essential residue class. For this class, a wide distribution of amino-acid types is observed after selection of the libraries with each of the β-lactam antibiotics, suggesting relaxed sequence requirements at these positions