In biology, the PYLIS downstream sequence (PYLIS: pyrrolysine insertion sequence) is a stem-loop

Stem-loop

Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...

 structure which appears on some mRNA sequences. This structural motif

Structural motif

In a chain-like biological molecule, such as a protein or nucleic acid, a structural motif is a supersecondary structure, which appears also in a variety of other molecules...

 causes the UAG (amber) stop codon

Stop codon

In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Proteins are based on polypeptides, which are unique sequences of amino acids. Most codons in messenger RNA correspond to the addition of an amino acid to a growing polypeptide...

 to be translated to the amino acid

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 pyrrolysine

Pyrrolysine

Pyrrolysine is a naturally occurring, genetically coded amino acid used by some methanogenic archaea and one known bacterium in enzymes that are part of their methane-producing metabolism. It is similar to lysine, but with an added pyrroline ring linked to the end of the lysine side chain...

 instead of ending the protein translation. In archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

the PYLIS downstream sequence is positioned straight after the UAG codon which is translated as pyrrolysine.