Both Sec and Pyl incorporation are examples of
Incorporation of twenty-first and twenty-second amino acids demonstrates natural genetic code expansion where organisms evolved extra chemistry beyond twenty canonical residues. Expansion requires novel amino acids, dedicated tRNAs with nonstandard structures, specific synthetases, and recoding signals repurposing stop codons while preserving termination fidelity. Unlike attenuation, RNA editing, or post-translational modifications, expansion alters translation table itself, increasing repertoire with selenol nucleophile and pyrroline-imine for catalysis. Synthetic biologists exploit these precedents to engineer codes incorporating unnatural amino acids. Evidence suggests code not completely frozen, acquiring new assignments when selective advantage provided.
Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 6: Genetic Code Expansion by Sec and Pyl