Protein biosynthesis depends on the availability of ribosomes,which in turn relies on ribosomal RNA production.In eukaryotes,this process is carried out by RNA polymerase I(Pol I),a 14-subunit enzyme,the activity of which is a major determinant of cell growth.Here we present the crystal structure of Pol I from Saccharomyces cerevisiae at 3.0 A resolution.The Pol I structure shows a compact core with a wide DNA-binding cleft and a tightly anchored stalk.An extended loop mimics the DNA backbone in the cleft and may involved in regulating Pol I transcription.Subunit A12.2 extends from the A190 jaw to the active site and inserts a transcription elongationfactor TFIIS-like zinc ribbion into the nucleotide triphosphate entry pore,providing insight into the role of A12.2 in RNA cleavage and Pol I insensitivity to a-amanitin.The A49-A34.5 hetrodimer embraces subunit A135 through extended arms,thereby contacting and potentially regulating subunit A12.2.