A minority of genes in probably all organisms rely on "recoding" for translation of their mRNAs. In these cases, the rules for decoding are temporarily altered through the action of specific signals built into the mRNA sequences. Three classes are described. 1. Frameshifting at a particular site allows expression of a protein from an mRNA with overlapping open reading frames, often giving two protein products from one mRNA. 2. The meanings of code words are altered: specific stop codons can be redirected to encode selenocysteine, tryptophan, or glutamine. 3. Ribosomes can translate over coding gaps in mRNA. These novel mechanisms expand the repertoire of the genetic code and are at the heart of several regulatory schemes.
Gesteland et al. (1996) studied this question.