I don't think there are any known to be radically different. Note that it's possible that life arose several times but only the lineage of that one cell survived, perhaps? In any case, it's not like there's been no variation.
Now, here's a mystery (AFAIK): Just where is the genetic code stored? I seem to recall reading an article a while back about how while this seems like an easy question it's actually not known. I can't seem to find it at the moment, though. Anyone know more about this?
The mapping between base triplets and amino acids is encoded in the tRNAs. And the tRNAs themselves are of course again encoded in the DNA sequences used to produce them.
I'd say it's encoded in the combination of the tRNAs and the aminoacyl-tRNA synthetases, which load the appropriate amino acids on to the tRNAs. There's nothing in the tRNAs themselves which picks out a specific amino acid - that happens because the synthetases recognise specific amino acids and specific tRNAs.
The synthetases are also encoded in DNA, so the fundamental point, that the code is encoded in DNA, stands.
I mean, that's the obvious answer, but I recall reading a thing about why it wasn't actually that simple, how swapping out the tRNAs didn't actually have the effect you would expect. Like, obviously the anticodons match the codons, no problem there, but there was some complicated confusing thing about how the rest of the tRNA matched the amino acid. Trying to find it. Hoping someone knows what I'm talking about.
> Surprisingly, a perfect proteome is not a pre-requisite for cellular viability even in the context of human cells. Lant et al. demonstrated that a single tRNA mutant can lead to significant mistranslation in human cells [17]. This was accomplished by expressing an Ala accepting tRNAPro G3:U70 variant in HEK 293 cells. The authors visualized a rate of ~ 3% mistranslation using a novel green fluorescent protein (D129P) reporter that fluoresces in response to mistranslation at proline codons. In contrast to previous studies in yeast [18], human cells in culture did not mount a detectable heat-shock response and tolerated the mistranslation without apparent impact on cell viability.
The eLife paper you link to discusses errors in RNA synthesis (transcription), not in tRNA-dependent protein synthesis (translation). The abstract mentions “translation”, but only as an amplifier of transcription errors: each mRNA transcript is translated into proteins thousandfold, so every error in a single mRNA molecule will be present in thousands of protein molecules.
Translation errors also exist, and some people hypothesise that there is selective pressure on protein-coding genes to reduce this source of errors by selecting codons in a way that reduces the error rate (potentially by slowing down the polymerase). This results in something known as “codon bias” but so far there is no good evidence that codon bias has an actual effect on error rate (it does have an effect on correct protein folding), or is selected for (http://dx.doi.org/10.7554/eLife.27344, http://dx.doi.org/10.1371/journal.pgen.1006024).
My first two links are indeed about transcription! Rereading my comment in context now I'm realizing it was a bit unclear. I was responding to "the genetic code is rarely discussed as a potential source of error".
However, I believe my third link is about environmental stress triggering intentional mistranslation (mRNA to protein). From that paper's figure 3:
> Proteins arising from “statistical proteomes” have various folding and binding properties, resulting in phenotypic diversity in the host organism.
I haven't bothered to pull up the related references (5 obvious ones) to assess their strength though.
The papers you linked seem to be claiming that codon encoding preferences (which vary by gene category) are in fact due to (or merely correlated with?) GC content in mammalian genomes (as opposed to a number of other previously proposed mechanisms). This is surprising because individual tRNA abundance varies by cell state and type, so that would have been the obvious (but apparently wrong) explanation. It's doubly surprising because a number of single celled organisms utilize the mismatch between codon preference and tRNA availability in order to regulate protein translation, but these papers are claiming that's not a significant factor in mammals.
> This is surprising because individual tRNA abundance varies by cell state and type, so that would have been the obvious (but apparently wrong) explanation.
tRNA gene expression varies by cell state, but isoacceptor abundance is in fact very stable (at least in mammals). Meaning, if you have a set of tRNA genes which all code for, say, Ala_AGC, the sum of the gene expression of all these genes is relatively stable, even if their individual expression varies (http://dx.doi.org/10.1101/gr.176784.114l; full disclosure: I’m an author on this and one of the previously linked papers).
Why individual tRNA gene expression varies, and how the cell regulates the overall stability, is unclear (my personal pet theory is that secondary tRNA function as regulatory RNA, in the form of tRNA-derived fragments, causes the need to regulate tRNA genes, see e.g. http://dx.doi.org/10.1016/j.cell.2017.06.013).
> It's doubly surprising because a number of single celled organisms utilize the mismatch between codon preference and tRNA availability in order to regulate protein translation
It’s not that surprising: gene regulation happens fundamentally differently in eukaryotes and prokaryotes, and even differently in different classes of eukaryotes. The effective population size (= evolvability) and genome complexity seems to play a role here. Simply put, higher animals have much more powerful and precise ways of controlling gene expression (enhancers and histone control). Regulation at the translation level is comparatively slow and wasteful (it’s several steps further down the line of the gene->protein production process).
I don't think there are any known to be radically different. Note that it's possible that life arose several times but only the lineage of that one cell survived, perhaps? In any case, it's not like there's been no variation.
Now, here's a mystery (AFAIK): Just where is the genetic code stored? I seem to recall reading an article a while back about how while this seems like an easy question it's actually not known. I can't seem to find it at the moment, though. Anyone know more about this?