Epigenetic Heredity: Transfer RNA Has Gone to Pieces

All eukaryotic (and many bacterial) RNAs descend from larger precursors, some of them thousands of times larger than the final product and others just a few nucleotides longer. The genomes of eukaryotes are pastiches of both assembled modules and subsequently introduced mobile elements, and the resulting baroque design has led to a situation in which almost all transcripts require trimming. The most dramatic case, of course, is the interruption of almost all eukaryotic transcription units for messenger RNA by intervening sequences, a.k.a. introns. Not the ideal plan, you say? Perhaps, but we cannot play the evolutionary tapebackward towitness what forces were at play in the purifying selection that led to the adoption of these designs for the production of RNA. Adopted they were. Ingrowingmammaliancells,messengerRNAs(mRNAs) live for 6–24 hours on average, with someoutliers on either side of that temporal window. The mRNA encoding the protein c-myc in mammalian cells lives for only a few minutes, whereas mRNAs produced during oogenesis in some animals are stored for months prior to the gamete’s maturation. As regards ribosomal RNA, in many cells it is so stable that its lifetime cannot be measured. When RNAs have lived out their life (a time that appears to be stochastic for some, i.e., the probability of an RNA’s decay being unrelated to its posttranscriptional age, vs. others that are marked for a certain persistence) it has generally been thought that the degradation processes, while perhaps spatially directed to a degree (chewing in from either end or slicing at some hypervulnerable sites in the middle), would typically generate aWalpurgisNight of pieces.These have been conceptually assigned to a trash bin. Alas, we now have a surprise. At least one class ofmature (processed) RNAs, eventually going to pieces, produces some functional fragments.