ABSTRACT
The biochemical properties o f the TRAMP complex and the nuclear exosome are consistent with much o f the in vivo analysis of the tRNA nuclear surveillance pathway described above. Purification o f the TRAMP complex by TAP-tagging o f any o f the components yields a complex of Trf4, A irl or Air2 and Mtr4 with substantial RNA-dependent poly (A) polymerase activity, which is specific for incorporation o f adenine nucleotides4244 and which is estimated to have similar specific activity to that o f the classical yeast poly( A) polymerase, Pap 1.43 Trf4 is the catalytic subunit of the poly(A) polymerase activity, based on its homology to polymerases and the lack o f activity o f a variant Trf4-TAP complex carrying a mutation at the deduced catalytic center.43,44 Airl and Air2 are putative zinc knuckle RNA binding proteins that are each tighdy associated with Trf4 in vivo, an association that is required for poly(A) polymerase activity o f Trf4 in vitro42'44, and Mtr4 is an RNA helicase.38
