Crystal structure of the C17/25 subcomplex from Schizosaccharomyces pombe RNA polymerase III.

Eukaryotic RNA polymerase III (Pol III) is a multisubunit enzyme responsible for transcribing tRNA, 5S rRNA, and several small RNAs. Of the 17 subunits in Pol III, the C17 (Rpc17) and C25 (Rpc25) subunits form a stable subcomplex that protrudes from the core polymerase. In this study, we determined the crystal structure of the C17/25 subcomplex from Schizosaccharomyces pombe. The subcomplex adopts an elongated shape, and each subunit has two domains. The two subunits in the subcomplex are tightly packed and extensively interact, with a contact area of 2080 Å(2) . The overall conformation of S. pombe C17/25 is considerably different from the previously reported structure of C17/25 from Saccharomyces cerevisiae, with respect to the position of the C17 HRDC domain, a helix bundle essential for cell viability. In contrast, the S. pombe C17/25 structure is quite similar to those of the Pol II and archaeal counterparts, Rpb4/7 and RpoE/F, respectively, despite the low sequence similarity. A phylogenetic comparison of the C17 subunits among eukaryotes revealed that they can be classified into three groups, according to the length of the interdomain linker. S. pombe C17, as well as Rpb4 and RpoF, belongs to the largest group, with the short linker. On the other hand, S. cerevisiae C17 belongs to the smallest group, with the long linker, which probably enables the subcomplex to assume the alternative conformation.