Nuclear RNA metabolism undergoes a strict quality control. An important component in all of these processing/degradation events is the nuclear exosome and a new RNA decay-activating Trf4/Air2/Mtr4 polyadenylation (TRAMP) complex. TRAMP recognizes a variety of nuclear transcripts produced by all three RNA polymerases, adds short poly(A) tails to aberrant or unstable transcripts, forming a favourable substrate for the exosome. However, it is unclear, how the aberrant RNAs are identified by the ups tream acting TRAMP. Here I propose to investigate the molecular basis for the RNA selection by the TRAMP complex. Not only known RNA substrates will be characterized in detail, but also new RNA targets identified by microarray analysis will be assessed. Potential substrates will be probed for common features that may serve as a recognition signal for targeting by the TRAMP/exosome degradation pathway. We will also investigate the molecular differences between poly(A) tails added by TRAMP and by canonical poly(A) polymerase that results in degradation or stability, respectively. The role of RNA helicase Mtr4p that is not required for the polyadenylation activity of TRAMP will be investigated. Structures of TRAMP and its subunits alone and in complex with RNA will be studied.