by the non-canonical poly(A) polymerase PAPD5, and the short oligo(A) tail added by PAPD5 is a marker for 3′ end processing by the ribonuclease PARN/EXOSC10 or for degradation by the exonuclease DIS3L. Since PARN deficiency causes a severe form of the bone marrow disease Dyskeratosis Congenita as well as pulmonary fibrosis, it is possible that defects in Y RNA processing contribute to the severe pathology observed in these patients. Species distribution Presumptive Y RNA and Ro protein homologs have been found in eukaryotes and bacteria. Humans Humans appear to have four Y RNAs, named hY1, hY3, hY4 and hY5 and also a large number of pseudogenes. C. elegans Caenorhabditis elegans has one, named CeY RNA and a large number of sbRNAs that are postulated to also be Y RNA homologues. D. radiodurans The radiation-resistant bacterium Deinococcus radiodurans encodes a homolog of Ro called rsr ("Ro sixty related"), and at least four small RNAs accumulate in Deinococcus under conditions where rsr expression is induced (UV irradiation); one of these RNAs appears to be a Y RNA homolog. In Deinococcus radiodurans Rsr is tethered via Y RNA to the exoribonuclease PNPase and channels single-stranded RNA into the PNPase cavity. Rsr and Y RNA enhance degradation of structured RNAs by PNPase. This role could be conserved, as Rsr and ncRNAs called YrlA and YrlB (Y RNA like) also associate with PNPase in an evolutionary distant bacterium Salmonella Typhimurium. References External links Non-coding RNA