Can pseudogenes be transcribed into mRNA and translated into functional proteins, or should they be regarded as functionless mutated genes?
- A subset of pseudogenes can have a function - even if not translated: For one possibility see the official record for PTENP1, which acts as a tumor suppressor by providing a decoy for PTEN targeting miRNAs
- A subset of pseudogenes can be translated. There is a selection for preserving the length of the reading frame, which suggests (but does not formally show) the presence of a translation-dependent function of (at least a subset of) those pseudogenes. - See Xu et Zhang 2016: Are human translated Pseudogenes Functional? (paywall)
- As indicated by the above, it seems likely that quite some pseudogenes have a function, which just has not been discovered yet - and that the mode of action can differ between different pseudogenes.
The term ‘pseudogene’ was originally coined to described genomic sequences related to protein-coding genes, but which contained:
“defects… which would completely prevent the production of functional protein.”
(Quoted from review by Jeffreys and Harris, 1984.)
There are two distinct types of pseudogene:
As their name implies, these are produced by gene duplications. As illustrated in the globin gene families, some of such duplications produce new functional genes, whereas others acquire inactivating mutations. For example a pseudogene with a premature stop codon might be transcribed and translated, but the protein would be truncated and probably degraded.
It should be noted that single mutations can, in the course of time revert, and there are examples of multi-gene families in which genes are active in some species and inactive in others. I have referred to the ABO gene family in this respect in an answer to another SE question.
These arise from reverse transcription of mRNAs into DNA which is inserted into the genome, presumably by enzymes encoded by transposable elements. This may occur in any cell, but when this occurs in non-somatic cells do they become established. Hence such pseudogenes tend to be for general genes such as non-muscle actins. They are recognizable by their lack of introns, polyA sequences, and lack of promoters. Unless, by chance, they should come under the influence of a promoter they will not be transcribed and, hence, not translated.