How Does Polycistronic mRNA Produce Multiple Polypeptides?

[Dr. Nick]

Hi everybody !

I wonder, how do 2, 3, 4 or more :) different polypeptides arise from one polycistronic mRNA, when they are spaced with only one or two base pairs ? I’m referring here to mitochondrial mRNA (or prokaryote DNA).
Which tool or mechanism recognizes this base pare as spacers and cuts polypeptide chain into more then one ?

 

[Dr. Nick]

Since no one answered my first question, here’s the second, was my question to much elementary or easy so no one bothers, or no one knows the answer?

Maybe you can help me to find a place were I can answer it?

Thanks,
Bye

 

[iansmith]

polycistronic mRNA in bacteria and eucarya processing is different. In bacteria, only a ribosomal binding site is required in front of the second, third and ect open reading frame(s). For eucaryotes, it is a bit more complex.

*EUKARYOTIC* polycistronic translation:

Polycistronic translation in eukaryotes can occur by a few mechanisms.
One way to achieve multiple proteins from a single polycistronic mRNA is
for the first start codon (AUG) to be in a poor context for translational
initiation. This means that due to structural configurations of the mRNA
(determined by mRNA sequence), some ribosomes will miss the first start
codon and slide down the message to the second start codon, at which they
will intitiate translation. Remember that the ribosome binds somewhere
upstream of the translation initiation point and slides down the
transcript until it encounters an AUG codon. The upstream binding of
ribosomes is a little different in eukaryotes than it is in prokaryotes.
Eukaryotic mRNA is usually capped at the 5' end somewhere after
transcription but before translation (post-transcriptional mRNA
processing) and this 5' cap binds to factors that increase the chance that
a ribosome will attatch to the transcript in this area.

...

A second mechanism to achieve multiple proteins from a polycistronic mRNA
is to have an internal ribosome entry site (IRES) after the first gene
transcript but before the second gene transcript. The ribosome will enter
the mRNA here and slide down the transcript until it reaches an AUG codon.

A third (and not well understood) mechanism involves the enhancement of
ribosomes binding to the mRNA downstream from a point of translational
termination. This is mechanistically different from the second mechanism
because here it is the same ribosome that transcribes both gene
transcripts, briefly hopping off the first and then back on before the
second. In the previous mechanism, any ribosome may enter at the IRES
without having previously translated that particular mRNA.

http://www.madsci.org/posts/archives/mar2001/985920332.Mb.r.html

 

[Dr. Nick]

Thank you iansmith,
So can I conclude that same mechanism applies for prokaryotes and mitochondria (only ribosomal binding site is required in front of the second, third and ect open reading frame(s)) ?

 

[iansmith]

As far as I know, the mitochondiral polycistronic mRNA is often process into mRNA that encode only one protein. So mitochondrial polycistronic mRNA is not often process as one unit.

Mitochondria polycistronic could be translated by ribosome using a similar mechanism (only need a RBS) that the one found in bacteria since the rRNA is quite similar. I have not found any information regarding this mechanism.

 

[Dr. Nick]

I came to this question during one lecture, in which one of my professors sad that most of 13 recognizable genes that code for polypeptides in mtDNA are transcribed into polycistronic mRNA, encoding parts spaced by only 1-2 bp.
I’ve asked her the same question but I didn’t get answer (she just repeated part about organization of mtDNA …

 

[Dr. Nick]

As far as I know, the mitochondiral polycistronic mRNA is often process into mRNA that encode only one protein. So mitochondrial polycistronic mRNA is not often process as one unit.

Mitochondria polycistronic could be translated by ribosome using a similar mechanism (only need a RBS) that the one found in bacteria since the rRNA is quite similar. I have not found any information regarding this mechanism.

Oh, so it goes like that, that makes sense.
But how is mRNA processed into smaller fragments, restriction enzyme, maybe some kind of splice mechanism, or something else ?

Regards!

 

[iansmith]

The mRNA is trans-spliced with a specialized spliced leader (SL), SL2.

You might be interrested in this chapter
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=ce2.section.165