The RNA exosome is responsible for a wide variety of RNA processing and 
degradation reactions. The activity and specificity of the RNA exosome is 
thought to be controlled by a number of cofactors. Mtr4 is an essential 
RNA-dependent adenosine triphosphatase that is required for all of the nuclear 
functions of the RNA exosome. The crystal structure of Mtr4 uncovered a domain 
that is conserved in the RNA exosome cofactors Mtr4 and Ski2 but not in other 
helicases, suggesting it has an important role related to exosome activation. 
Rrp6 provides the nuclear exosome with one of its three nuclease activities, and 
previous findings suggested that the arch domain is specifically required for 
Rrp6 functions. Here, we report that the genetic interactions between the arch 
domain of Mtr4 and Rrp6 cannot be explained by the arch domain solely acting in 
Rrp6-dependent processing reactions. Specifically, we show that the arch domain 
is not required for all Rrp6 functions, and that the arch domain also functions 
independently of Rrp6. Finally, we show that the arch domain of Ski2, the 
cytoplasmic counterpart of Mtr4, is required for Ski2's function, thereby 
confirming that the arch domains of these cofactors function independently of 
Rrp6.