miRNA targets in comparison to siRNA targets - (Nov/22/2006 )

Hello ...

I just started looking into RNAi, so I am a newbie ...
AFAIK both siRNA and miRNA have their targets on the 3'UTR side of the
mRNA.
But now I read, that siRNA Designers try to find specific sequences for
one mRNA to "knock out".
Is it unimportant where the exact target in the 3'UTR region of the
mRNA is located?
Is this also true for miRNAs?
Then I read some publications with contradictorily statements. One
says, that the siRNA could only cleave the mRNA (destroy it) and miRNA
could only cease translation. Then another predicates miRNA could do
both, and another publication both of them could do both.
What's the actual opinion about that?

Best regards,
coccus

Hello coccus,
The biggest difference is their biogenesis. Apart from that, they are very similar.
If a microRNA has a perfect matched target on a mRNA, it can also cleave the target like an siRNA does.
microRNAs not only inhbit protein translation, but also affect mRNA levels (which I guess is even more profound than translation inhibition) . Many microarray experiments have shown that knocking down microRNAs or miR processing genes, their target mRNAs will increase in levels.
However, there are still many known unknowns. we will wait and see...

In general, miRNAs are endogenous while siRNAs are regarded as the dsRNA products of RNAi( though there exists siRNA in vivo). however, there r so many similarities between them in functionanl mechanism that you can't distinguish them exactly and i think they might converge together or have sth in common some time. Who knows?!