Transcription and RNA Processing

ID #1189

I have a question that is really simple and probably stupid, but do prokaryotes have a 5' cap and a Poly-A tail and if not how do they protect against RNases?

This isn't a stupid question at all -- it's the next logical question to ask based on what we've discussed. If you are told that eukaryotic mature mRNAs are "protected" to a certain extent by their caps and tails, and you're also told that bacterial mRNAs don't have caps and tails, you might (and obviously do) wonder if that makes them more "endangered." That is in fact the case, but that's not always such a bad thing. Bear with me on this. Life evolves the way it needs to evolve to continue, which means in this case that if bacterial mRNAs needed that extra protection, they'd probably have it. Conversely, you could also say that if eukaryotic mRNAs didn't need the protection, they'd probably not have it (and would have evolved some other method of AUG selection). Now, if we examine the "life expectancy" of a bacterial mRNA, we'll see that it's on the order of seconds to minutes -- precisely because they're subject to quick degradation. Eukaryotic mRNAs, on the other hand, will typically last on the order of hours or even days.

Why is this beneficial to each? It boils down to adapting to a change in environment. A bacterial cell's ribosomes will continue to make protein from a mRNA as long as it's around. If all of a sudden, the bacteria needs to divert its resources to making a different set of proteins, it no longer wants to spend the energy to make protein from that "old" mRNA, but the ribosome doesn't know the difference -- it's just looking for a signal. And single celled organisms find themselves in rapidly changing environments, so it's in their best interest to be able to make quick changes. Yes, they'll have to transcribe a given gene more frequently because the mRNA is getting degraded, but in real time, you'd be amazed how much protein you can make from a mRNA in a few minutes.

Eukaryotic cells that are part of a larger multicellular organism like a human, on the other hand, don't find themselves in conditions that typically change as drastically as what a bacterium might encounter. They stay in the same place, do the same job, etc. So it is to their advantage to keep mRNAs around longer, since there's a much higher likelihood that they're going to need to keep making that protein for a while.

See how each has evolved to be as efficient as it can be under its own conditions?

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