1) the instructions for making the protein are transcribed from a DNA molecule to a molecule of messenger RNA (mRNA);If you want more detail than that, the British Society for Cell Biology has it.
2) the mRNA molecule carries the instructions to a ribosome;
3) the ribosome assembles the protein.
From the time that we first discovered what ribosomes do, scientists have imagined taking over this amazing machinery to make custom proteins or other molecules. You can get cells to make new proteins by altering their DNA, the way we get yeast to make vanilla and so on. But this only works if you can find a gene for that protein that will work in your target cell's genome. This excludes many proteins that one might like to make, including completely novel, unnatural ones. Which is why there is continued interest in directly hacking the ribosome.
So for a decade now scientists have tried creating mutant ribosomes and inserting them into cells, hoping that the cells would then produce the desired proteins. But because the two halves come apart and recombine with others, the hacked ribosomes kept combining with native ribosomes in failed combinations that could make neither the natural nor the hacked proteins, and the cells stopped growing or even died.
And now this week's news:
The solution, Mankin and Jewett's team decided, was to marry together two engineered subunits. It was unclear whether the approach would work: it was thought that ribosomes exist in two distinct units because it is necessary for their function.
The researchers used a strand of RNA to tether the large and the small subunit together, toiling for months to get the length and location of the link just right so that the machine could still function. “We certainly came close, several times, to saying ‘OK, biology wins',” says Jewett.
The team screened its tethered ribosomes in Escherichia coli cells that lacked functioning RNA, and eventually found engineered ribosomes that worked well enough to support some growth, albeit slow. They then tested their platform to confirm that a tethered ribosome could operate side-by-side with natural ribosomes.
The result unlocks a molecular playground for bioengineers: by tethering the artificial subunits together, they can tweak the engineered machines to their liking without halting cell growth.