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"Resurrected" Flagella Were Just Unplugged

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A recent paper in Science carries the clickbait title "Evolutionary resurrection of flagellar motility via rewiring of the nitrogen regulation system." I can't blame scientists in these days of comparatively low funding for trying to attract attention to their work. But the public will eventually grow jaded if misled readers keep finding jazzy labels stuck to picayune results.

A related cheerleading story in The Scientist says that the workers didn't set out to investigate flagellar evolution. Rather, starting with the soil bacterium Pseudomonas fluorescens,which normally sports a fine, functioning flagellum, they intentionally deleted just the gene for the master switch protein (dubbed "FleQ") that controls flagellum development in order to see how the bug might deal with immobility while colonizing plants. (The genes for the dozens of other protein parts needed for the flagellum were all left intact.) They were surprised to see that the bacteria regained the ability to make flagella after being incubated on Petri dishes for a few days.

Their investigation of the once-again-mobile P. fluorescens showed a couple of genetic changes. Most frequently a point mutation (D228A) appeared in a protein called NtrB. That mutation had previously been shown to cause the protein to stay on continuously (like breaking the controls of a chain saw so it can't be switched off). The ordinary role of NtrB is to chemically modify another protein, NtrC, under appropriate circumstance, which activates it. The mutant, constitutively active, unregulated NtrB kept NtrC continually in its active form, too.

Now, like FleQ (the master regulator switch for flagellar genes), active NtrC is itself a master switch that usually turns on genes involved in another pathway -- nitrogen metabolism. It also turns out to be a homolog of FleQ. In other words, the two switch proteins were already structurally very similar. Apparently the unmutated NtrC already had some ability to cross-bind to the DNA control region that FleQ usually bound to, allowing the extra NtrC -- produced when its regulator was broken -- to flip the switch which turned on the pre-existing flagellum biosynthesis pathway.

That mutation turned on the flagellum pathway somewhat less effectively than occurs in the original, unmutated bacteria (ones that have intact FleQ). A subsequent mutation in the gene for the DNA-binding region of NtrC increased its ability to turn on the flagellum pathway to greater-than-normal. The authors suggest (but they didn't investigate) that the second mutation helped NtrC bind more tightly to the flagellum control region. Maybe so, but there's a fly in the ointment. A NtrC mutant that lost its entire DNA binding region also helped promote flagellum synthesis. Something funny going on there.

Some other changes could take the place of the initial mutation in NtrB. Mutations that cause the loss of function of several other genes involved in nitrogen metabolism have the effect of keeping NtrB turned on all the time, too, which jacks up the concentration of NtrC and leads to the same result concerning flagella. Meanwhile, with the mutations in NtrB and NtrC, the poor bug lost the ability to control its genes for nitrogen metabolism. As a participant in the study said, "The bacteria that became much better at swimming were much worse at nitrogen regulation." But "sometimes the advantage can be so great that it's worth paying that cost because otherwise you die."

Here's an analogy for the work's relevance to the evolution of the flagellum. Suppose some guy told you that natural processes could make a functioning television. Intrigued, you say great, show me. He takes a thousand working TV sets, unplugs them, and places the plugs next to electrical outlets. Eventually a strong wind comes along and blows one of the plugs into contact with the outlet, turning a TV on. "See!" he exclaims.� "What'd I tell you?! Natural process, functioning TV!"

Hmm. Maybe The Scientist would describe such results as a "giant evolutionary leap" as they did with the flagellum paper, but I think most people would be decidedly unimpressed.

To recap, the first step in the path was a loss-of-function mutation, either directly to NtrB or to proteins that have the effect of keeping NtrB continually active. The second step was a mutation to NtrC, whose exact effect is unknown. These trigger the expression of a very complex, pre-existing pathway involving several dozen pre-existing proteins, leading to flagellum development. I myself would not characterize that as an "Evolutionary Resurrection." But I guess a paper titled "Crippled Bacteria Forced to Pay Heavy Evolutionary Cost Lest They Die" wouldn't wind up in Science.

Image: Pseudomonas fluorescens, by Ninjatacoshell (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons.