The RRP42 subunit of the archaeal exosome is a member of the RNase_PH family, named after the bacterial Ribonuclease PH, a 3'-5' exoribonuclease. Structurally all members of this family form hexameric rings (trimers of dimers). In archaea, the ring is formed by three Rrp41:Rrp42 dimers. The central chamber within the ring contains three phosphorolytic active sites located in an Rrp41 pocket at the interface between Rrp42 and Rrp41. The ring is capped by three copies of Rrp4 and/or Csl4 which contain putative RNA interaction domains. The archaeal exosome degrades single-stranded RNA (ssRNA) in the 3'-5' direction, but also can catalyze the reverse reaction of adding nucleoside diphosphates to the 3'-end of RNA which has been shown to lead to the formation of poly-A-rich tails on RNA. It is required for 3' processing of the 5.8S rRNA.
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