= SecM arrest sequence
{c}
The <SecM arrest sequence> is a nascent-peptide motif that can cause <ribosome stalling> during synthesis of <SecM>. A2002 study reported the pattern `FXXXXWIXXXXGIRAGP`, including a specific arrest point associated with the terminal <proline>. The functional interaction occurs while the chain is being translated within the <ribosomal exit tunnel>.<The ribosomal exit tunnel functions as a discriminating gate by Hitoshi Nakatogawa and Koreaki Ito>[Primary Abstract: motif, arrest point and tunnel-dependent elongation arrest]{ref}
An original indexing table makes the notation unambiguous:
$$
\begin{array}{c|rrrrrrrrr}
\text{position in motif}&1&6&7&12&13&14&15&16&17\\
\text{specified residue}&F&W&I&G&I&R&A&G&P
\end{array}
$$
The remaining eight positions are represented by X. These are positions within the17-residue motif, rather than asserted numbering in every full-length <protein>. X denotes an unspecified <amino acid residue>, not an actual amino acid called X.
If each X were allowed any of the20 standard <amino acids>, a purely combinatorial pattern would match $20^8=25{,}600{,}000{,}000$ strings. That count does not establish that every one produces the same biological arrest. A sequence pattern, an experimentally competent peptide and its ribosomal context carry different information.
The original experiment also found bypass mutations in <23S ribosomal RNA> and <ribosomal protein L22> at tunnel-facing positions. This supplied evidence that both the emerging chain and the receiving molecular environment matter. The export machinery's pulling on the N-terminal part of <SecM> can relieve stalling, connecting the pause with progress outside the ribosome.<The ribosomal exit tunnel functions as a discriminating gate by Hitoshi Nakatogawa and Koreaki Ito>[Abstract: suppressor locations and export-dependent pulling]{ref}
An illuminating comparison would hold the peptide pattern fixed while changing a tunnel component, then test a control peptide in the same backgrounds. A changed arrested fraction would identify a context-dependent response; it would not mean that the motif's letters had changed. Such comparisons help distinguish sequence recognition from a universal stop command in the <genetic code>.
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