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!!RNA nerd thread!! This amazingly cool paper was published in the last few (amazingly chaotic) days, showing that a small RNA produced by a symbiotic bacterium dramatically affects the biology of its host squid https://journals.plos.org/plosbiology/article?id=10.1371%2Fjournal.pbio.3000934
This small RNA (called 10Sa or SsrA or tmRNA) has a neat history. It was first discovered as a small stable RNA in E. coli in 1978.
It was called “10S” because of its sedimentation through a sucrose gradient (the same way that bacterial ribosomes are called 70S). It’s function was unknown. https://jb.asm.org/content/133/2/1015.short
The gene for 10Sa RNA was eventually mapped on the E. coli genome, and it was named ssrA (for small stable RNA). At that point (in 1990), it was known to be present in about 1000 copies per cell, but the function was still unknown. https://jb.asm.org/content/172/8/4708
The first hint of a function came in 1994, when it was reported that part of the 10Sa/SsarA RNA folded into a tRNA-like structure and could be charged with alanine in vitro(!).
https://www.pnas.org/content/91/20/9223.short
https://www.pnas.org/content/91/20/9223.short
Another clue to function came the next year, in 1995, when a group trying to make recombinant mouse interlukin-6 in E. coli noticed that a significant fraction of the proteins produced in the cell had an 11 amino acid extension at the C-terminus.
This 11 amino acid extension, it turns out, corresponded *exactly* to a segment of the ssrA gene. https://www.jbc.org/content/270/16/9322.full
Finally, in 1996, Bob Sauer and colleagues put it all together and showed that what 10Sa RNA was doing was rescuing stalled ribosomes. For example, if an mRNA is broken and loses its stop codon, the ribosome will stall at the end of the message. https://science.sciencemag.org/content/271/5251/990.abstract
If 10Sa RNA is around, the stalled ribosome will SWITCH TO A NEW RNA MOLECULE (the 10Sa) by recognizing the tRNA-shaped part of 10Sa, then treat the rest of 10Sa RNA as an mRNA, add the 11 amino acid signal, and then terminate normally to translate again.
That 11-amino acid signal targets the protein for degradation. THIS IS BANANAS!
Why do I care so much about this little RNA? Because Sauer’s paper was published about the time I was trying to figure out what to work in grad school, and I loved it. (It also so happens that my very first paper was on the structure of 10Sa RNA)
https://rnajournal.cshlp.org/content/3/1/89.short
https://rnajournal.cshlp.org/content/3/1/89.short
So seeing this little RNA now being involved in the squid-Vibrio symbiosis makes me very very happy. Thanks for reading, RNA nerds!
