We report a bacterial system for the evolution of cyclic peptides
that makes use of an expanded set of amino acid building blocks.
Orthogonal aminoacyl-tRNA synthetase∕tRNACUA pairs, together
with a split intein system were used to biosynthesize a library of
ribosomal peptides containing amino acids with unique structures
and reactivities. This peptide library was subsequently used to
evolve an inhibitor of HIV protease using a selection based on cellular
viability. Two of three cyclic peptides isolated after two rounds
of selection contained the keto amino acid p-benzoylphenylalanine
(pBzF). The most potent peptide (G12: GIXVSL; X ¼ pBzF) inhibited
HIV protease through the formation of a covalent Schiff base
adduct of the pBzF residue with the ϵ-amino group of Lys 14 on
the protease. This result suggests that an expanded genetic code
can confer an evolutionary advantage in response to selective
pressure. Moreover, the combination of natural evolutionary processes
with chemically biased building blocks provides another
strategy for the generation of biologically active peptides using
microbial systems.