Molecular Genetics

Signals guarantee that translation products of incomplete mRNAs do not accumulate.

Facts

• Many bacteria have an RNA, tmRNA, that can be charged with alanine (ref) by alanyl tRNA synthetase.
• tmRNA has a short open reading frame that ends with the codons for ala.asn.asp.glu.asn.tyr.ala.leu.ala.ala (ANDENYALAA).
• In E. coli, overespression of the gene for interleukin led to the synthesis of some polypeptides that terminated with AANDENYALAA (ref).
• The gene that encodes a precursor to tmRNA, ssrA is required for the AANDENYALAA tagging.
• Stop codon-lacking mRNAs used in in vitro translation in E. coli systems, such as  polyU  (ref) and in vitro transcripts of plasmids linearized at sites preceding stop codons (ref) also result in the addition of AANDENYALAA.
• Bacteria have proteases that act on polypeptides containing C-terminal non-polar sequences, such as YALAA.

Interpretations

• tmRNA is both a tRNA and an mRNA.
• The polypeptide made is encoded by two mRNAs and thus by two genes (two genes-one polypeptide).
• These reactions ensure that translation products of incomplete mRNAs do not accumulate.
• 3' non-translated regions are required for proper function of mRNAs.

Further information

• The translation of tmRNA is called trans translation.  A database of sequences is available.
• Complete translation products are degraded by other means.
• tmRNA may play a role in regulation of gene expression (rev).  Binding of lac repressor to lac operator causes truncation of the lacI RNA and addition of the tag to lacI peptides (ref).

This is page 2551 of Molecular Genetics by Ulrich Melcher, © 1998, 1999, 2002