For example, mutations in the Naf1 protein, which is re- quired for the assembly of H/ACA snoRNPs, lead to the loss of that class of snoRNAs (Fatica et al. 2002; Dez et al. 2002). Similarly, mutations in the SM-binding sites of yeast U1, U2, and U5 or the telomerase RNA and TLC1 lead to decreased levels of these RNAs (Jones and Guthrie 1990; Seipelt et al. 1999; Seto et al. 1999). Although effects on transcription have not been ruled out, the simplest model is that the failure to properly assemble the snoRNP/snRNPs leads to their accelerated degradation by a yet-to-be-determined degradation pathway. Defects in U6 snRNA biogenesis also give increased decay of RNA. For example, an internal deletion within the U6 snRNA leads to unstable transcripts, which are adenylated and degraded by the nuclear exosome (Kadaba et al. 2006). Similarly, defects in the Lsm2–8 complex, which binds the 39 end of the U6 snRNA, lead to decreased levels of the U6 snRNA (Pannone et al. 2001, 1998; Luhtala and Parker 2009), although the mechanism of U6 degradation in this case is not known. Taken together, these observations argue that systems exist to degrade snRNAs and snoRNAs that fail to be properly assembled into RNPs. Degradation of RNAs in Mitochondria tRNAs, rRNAs, and mRNAs are also transcribed within the mitochondria of yeast cells in 13 complex transcription units. These transcripts are subject to RNA processing, including splicing of group I and group II introns, endonuclease cleavage, and 59 and 39 trimming. The biogenesis of mito- chondrial mRNAs typically involves a 59 endonucleolytic cleavage, often followed by 59 to 39 trimming of the exposed 59 end to speciﬁc sites and a 39 endonuclease cleavage just downstream of a so-called dodecamer element (Dieckmann and Staples 1994). The dodecamer element is proposed to bind an unknown protein factor and may play a role in trans- lation and/or stabilization of the mRNAs (reviewed in Gagliardi et al. 2004). In contrast to cytoplasmic mRNAs, mitochondrial mRNAs in yeast are not polyadenylated. The major functional RNAse in the mitochondria mech- anism is termed the mitochondrial degradasome or mtEXO and consists of a 1:1 ratio of Dss1, a member of the RNaseII family, and Suv3, a DEVH family member. Strains defective in Suv3 or Dss1 show accumulation of mitochondrial mRNAs, indicating a defect in their turnover (Dziembowski et al. 2003; Malecki et al. 2008). Suv3 and Dss1 mutants also accumulate excised introns, indicating that mtEXO is required for their degradation (Szczesny et al. 2011). Interestingly, such strains also accumulate incompletely pro- cessed mitochondrial rRNAs, which has led to the sugges- tion that the mtEXO also functions in a quality control system for the degradation of rRNAs or mRNAs that are not completely processed. The activity of the mtEXO on mitochondrial mRNAs may be limited by the 39 features of mRNAs, possibly including a factor binding the dodecamer element (Min and Zassenhaus 1993). 692 R. Parker The activity of mtEXO is due to the concerted action of the Suv3 helicase activity and the Dss1 nuclease in response to a RNA substrate. For example, Suv3 has intrinsic ATPase activity, but, in a complex with Dss1, the ATPase actitivy is enhanced by a single-stranded nucleic acid substrate (Malecki et al. 2007). Moreover, the helicase activity of Suv3 is dependent on Dss1 (Malecki et al. 2007). Similarly, the exonuclease activity of Dss1 is increased by Suv3 and made dependent on ATP (Malecki et al. 2007). These obser- vations suggest a working model whereby Suv3 acts as an ATP-dependent motor to feed RNA into the active site of Dss1 (Szczesny et al. 2011). There is also a 59 to 39 exonuclease activity in mitochon- dria that is dependent on the Pet127 protein (Wiesenberger and Fox 1997; Fekete et al. 2008), although whether Pet127 is actually a nuclease remains to be determined. The Pet127- dependent 59 to 39 exonuclease may also be able to stimu- late and/or degrade mitochondrial RNAs since dominant mutations in, or overexpression of, Pet127 partially sup- presses the defects seen in suv3 or dss1 mutants (Wegierski et al. 1998; Chen et al. 1999). Strains lacking Pet127 also show misregulation of some mitochondrial mRNAs, although whether this is due to misprocessing of their 59 ends or due to disruption of normal mRNA degradation pathways is not yet clear (Wiesenberger and Fox 1997; Fekete et al. 2008). Two additional nucleases have been localized to mito- chondria but do not appear to affect RNA degradation. These include Nuc1, which is the major DNAse/RNAse in mitochondria by enzymatic activity (Dake et al. 1988; Vincent et al. 1988), and Rex2, which is a 39 to 59 exonucle- ase that is found both in the mitochondria (Hanekamp and Thorsness 1999) and in the nucleus, where it plays a role in the trimming of 5S and 5.8S rRNAs and U4 and U5 snRNA (Van Hoof et al. 2000a). Although no clear role for these nucleases has been identiﬁed in yeast mitochondrial RNA degradation, it is notable that nuc1Δ and rex2Δ strains show a negative genetic interaction for growth, suggesting some degree of functional overlap (Costanzo et al. 2010). Future Perspectives There are several areas of importance in the future study of RNA turnover in yeast cells. For example, although many RNA degradation pathways, enzymes, and cofactors have been identiﬁed, a precise understanding of their biochemical function and mechanisms of action will need further exper- imentation, including the development of robust in vitro sys- tems for mechanistic studies. Moreover, one anticipates that there are yet-to-be-identiﬁed nucleases that play roles in RNA degradation. It will also be important to understand how RNA degradation systems interface with other cellular processes. Although this is a general issue, three examples stand out. First, the tight inverse coupling between translation and mRNA degradation highlights that an understanding of the regulation of mRNA function will require mechanistic