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Bryan Cullen and colleagues at Duke University and the University of Kansas investigated the development of human micro-RNAs (miRNAs) to find that their precursors, primary miRNAs (pri-miRNAs), aren?t that different from protein coding mRNAs. Looking at nine of these pri-miRNAs they found poly adenylated tails and 5' caps, and evidence suggests that they?re transcribed by RNA polymerase II.
One of the subjects, miR-21, which is 22 nucleotides long, is processed out of a primary transcript more than 3,400 nucleotides long and intronless. ?People weren?t thinking in terms of long transcripts turning into small ones, says Cullen. The precursor to miR-21 even proved able to produce functional mRNA in transfected cells.
John McCarthy, a geneticist at the...