Genetics question

[PhantomfanMD]

On my final exam today there was a question about how proteins in E. Coli are made and one of the choices was:

"Proteins are made as one big protein and proteolytic enzymes break the big protein into smaller proteins."

Is that the correct choice? Or is this more common of an occurrence in Eukaryotes?

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[akinf]

E. coli is polycistronic, therefore it can produce multiple mRNA units from the same gene to produce different proteins. I think the use of proteolytic enzymes to break big proteins is a eukaryotic thing.

Correct me if I'm wrong.

 

[Samus]

It's a very common thing in translation of proteins in viruses (eg. retroviruses).

Here's a link that might help:
http://www.ncbi.nlm.nih.gov/entrez/...+rv[book]+AND+115308[uid]&rid=rv.chapter.2494

 

[midn]

We'd need a bit more context to answer that question. There is proteolytic activity in all protein formation to some extent since all proteins have some sort of ID tag stuck to them (the exact name of them escapes me) which is read by the cell and used to transport the protein to its correct place. Once the protein has been transported, the ID tag is sometimes cut off.

If the answer is talking about the vast majority of protein in E. coli, then I would say it is incorrect. There are several instances where larger protein are cleaved into smaller functional units of protein (like the translation of DnaX to produce a larger protein which is cleaved into the gamma and tau proteins for DNA polymerase), but from what I remember, most of the protein are directly translated and then folded.

Once again, I must add that you'd need to give us more context to the question. There may have been better answers.

 

[heymanooh1]

E. coli is polycistronic, therefore it can produce multiple mRNA units from the same gene to produce different proteins.Correct me if I'm wrong.

You are wrong. A prokaryotic polycistron can have several genes within it but the expression of those genes are under the control of one or more regulatory elements. When an mRNA is synthesized for the polycistron, it is as a single mRNA but with several opening reading frames/start codon sites leading to the different gene products.

On my final exam today there was a question about how proteins in E. Coli are made and one of the choices was:

"Proteins are made as one big protein and proteolytic enzymes break the big protein into smaller proteins."

Is that the correct choice? Or is this more common of an occurrence in Eukaryotes?

For E.coli think about the lac operon as an example. Prokaryotes, in general, make polycistronic mRNAs. The prokaryotic polycistronic mRNA will have several translation initiation sites unlike, generally, the monocistronic eukaryotic counterpart which will have a single start codon. A polycistron is advantageous to E.coli because it would want to respond to changes in the environment quickly, e.g. using lactose instead of glucose as its carbon source.

Generally, in eukaryotes the translated product is not the mature protein. For example, insulin is synthesized as inactive pro-insulin prior to post- translational modification. As such, there tends to be more extensive post-translational modification in eukaryotes than in prokaryotes in generating a functional/active/mature protein.

Maybe the question is basically asking the general features in prokaryotes/eukaryotes leading to a functional protein(s) once the mRNA is made.

 

[rcd]

On my final exam today there was a question about how proteins in E. Coli are made and one of the choices was:

"Proteins are made as one big protein and proteolytic enzymes break the big protein into smaller proteins."

Is that the correct choice? Or is this more common of an occurrence in Eukaryotes?

Hope you did well on the other questions .

That mechanism of protein synthesis happens, but it's an oddity.


The correct answer is along the lines of "ribosomes bind to specific ribosome-binding sites (each gene has 1 binding site) on the mRNA, translate until reaching a stop codon, and then detach." So, the individual proteins on the polycistronic RNA are translated as separate, non-bound proteins.