The second structure of a eukaryotic RNA polymerase II so far determined, that of the enzyme from the fission yeast Schizosaccharomyces pombe, is reported here. Comparison with the previous structure of the enzyme from the budding yeast Saccharomyces cerevisiae reveals differences in regions implicated in start site selection and transcription factor interaction. These aspects of the transcription mechanism differ between S. pombe and S. cerevisiae, but are conserved between S. pombe and humans. Amino acid changes apparently responsible for the structural differences are also conserved between S. pombe and humans, suggesting that the S. pombe structure may be a good surrogate for that of the human enzyme. molecular replacement ͉ X-ray crystallography ͉ yeast X -ray crystal structures of RNA polymerases from bacteria and Archaea, and of RNA polymerase II (pol II) from the budding yeast Saccharomyces cerevisiae, have given insight into the fundamental mechanism of transcription (1-7). The structures are closely similar in the catalytic core of the enzyme, where mobile elements interact with nucleic acids during transcription. The bacterial and S. cerevisiae structures differ in the periphery, where the enzymes interact with accessory factors, most notably the general transcription factors and Mediator of eukaryotes. One of the general factors, transcription factor II B (TFIIB), exhibits a degree of conservation with bacterial sigma factor and has a counterpart in Archaea, and a subunit of another general factor, the TATA-binding protein, occurs in Archaea as well, but the remaining general factors and Mediator are unique to eukaryotes.Conserved elements of the catalytic core include ''switch regions'' at the base of a swinging clamp, which interact with the DNA-RNA hybrid and downstream DNA during transcription (2, 3) and may be involved in transcription start site determination (8, 9). A so-called ''trigger loop'' interacts with the substrate nucleoside triphosphate to achieve the high fidelity of transcription (10)(11)(12)(13)(14). A ''bridge helix'' interacts with the coding base in the template strand of the DNA and is believed to play a role in the translocation step of transcription.Peripheral, more divergent elements of pol II include subunits Rpb4 and Rpb7, which form a dimer protruding from the enzyme surface (15, 16) and may be involved in RNA binding. Nearby lies the C terminus of Rpb1, from which extends a flexible linker, followed by Ͼ20 repeats of a 7-aa sequence [the C-terminal domain (CTD)], which interact not only with Mediator but also with capping, splicing, and cleavage/polyadenylation factors during transcription (17).Pol II is 53% identical in amino acid sequence between S. cerevisiae and humans, assuring a high degree of structural similarity. Notable differences in transcription have nevertheless emerged, including differences in promoter structure and interactions with accessory factors. In these respects, the fission yeast Schizosaccharomyces pombe more closely resembles the human system....