miRNA turnover rate - how long are endogenous miRs 'viable' (Feb/26/2009 )

I had a quick question. How long are endogenous miRNAs active/viable in the cell?

In cultured cells, after application of a transcription blocker like Actinomycin D, or maybe after transient siRNA knockdown of DGCR8/Drosha/Dicer, how long does it take for the endogenous miRNAs to clear out before one should really start assaying for de-stabilized (or increased stability, however you look at it) mRNA transcripts?

Any ballpark/exact estimates about high and low copy number miRs?

This is a good question. It seems there is no solid data on this yet. Precusor miRNAs are known to be short lived and mature miRNAs are relatively stable. From knockdown experiments, when miR processing genes such as drosha, dicer are knocked down, we could see gene expression changes within 48-72 hrs or earlier.