Non-standard residue

This subsection of the ‘Sequence annotation (Features)’ section describes the occurrence of non-standard amino acids selenocysteine (Sec) or pyrrolysine (Pyl) in the protein sequence.

Selenocyteine and pyrrolysine are represented in the sequence using the one-letter codes ‘U’ for selenocysteine and ‘O’ for pyrrolysine.

Incorporation of a selenocysteine or of a pyrrolysine in the protein sequence is due to a stop codon read-through event by which the meaning of the stop codon UGA or UAG, respectively, is redefined during the process of translation.
Examples: P24183 (selenocysteine), Q8TTA5 (pyrrolysine).

This subsection is only present when the incorporation of the non-standard amino acid has been experimentally proven. It can be propagated ‘By similarity’ to closely related homologs.

If a selenocysteine is present in an enzyme where it is part of the active site (as in glutathione peroxidase or iodothyronine deiodinase), then the position is annotated both in the ‘Non-standard residue’ and ‘Active site’ subsections of the ‘Sequence annotation (Features)’ section.
Example: P07203

Related keyword:
In all entries whose sequence contains pyrrolysine: Pyrrolysine
In all entries whose sequence contains selenocysteine: Selenocysteine
In all entries describing proteins that bind or contain selenium, or whose function is selenium-dependent: Selenium