July 29, 2015 § 1 Comment
Frustration has its perks. In a paper just out in Nature, researchers describe making an artificial ribosome because they couldn’t get normal ribosomes to do what they wanted. In creating this artificial ribosome, called Ribo-T, the investigators unwittingly turned conventional molecular biology wisdom on its head: Unlike regular ribosomes, Ribo-T doesn’t need to fall apart and come together again to support protein synthesis.
Alexander Mankin from the University of Illinois at Chicago says his group and that of Michael Jewett at the Northwestern University were trying to teach normal ribosomes new tricks, like getting it to translate “difficult-to-make” proteins or to take in unnatural amino acids to make special polymers. “We were frustrated with our inability to test or alter the functions of the ribosome,” says Mankin.
Trying to tweak the existing ribosomal RNA, which does much of the work of protein synthesis in the ribosome, didn’t go anywhere. Changes to it killed the cell.
So Mankin, Jewett and their teams considered making a portion of the ribosome that would be able to guide the ribosome into making the special polymers. But the problem is that the ribosome, made up of two subunits, falls apart and comes together in every cycle of protein synthesis. How would they stop the re-engineered portion of the ribosome from being swapped out by the normal subunit?
That’s when the idea of a tether came in. But “dissociation of ribosomal subunits was believed to be a prerequisite for efficient translation, and it was unclear whether ribosome with the tethered subunits would be functional,” says Mankin. Still, the investigators decided to give it a shot.
After many tries, one design worked: the Ribo-T. Mankin, Jewett and colleagues engineered a ribosomal RNA that combined sequences from the two subunits of the ribosome into a single unit. Short RNA linkers separated the two subunit RNAs in the contiguous stretch of nucleic acid.
And Ribo-T worked even better than anticipated. Not only did Ribo-T make proteins in a test tube, it also made proteins in bacterial cells that lacked naturally occurring ribosomes and keep the cells alive. Mankin still sounds surprised: “We have created probably the first-ever-on-Earth organism which lived with the ribosome where two subunits are combined into a single entity.”
He adds that Ribo-T could pave the way to exploring properties of the ribosome and to make a independent protein-synthesis system in cells that does not interfere with the ribosomes that take care of expression the rest of the cellular proteins. But, for now, the investigators are focusing on what sparked off the whole project in the first place: Getting Ribo-T to carry out the tasks that are difficult for normal ribosomes to do.