11 Transcription of Vertebrate snRNA Genes and Related Genes

OVERVIEW Small nuclear RNAs (snRNAs) and other RNAs involved in RNA processing (collectively denoted pRNAs) are encoded by a family of genes with structurally related promoters. Yet, some of these genes are transcribed by RNA polymerase II (pol II) and others by RNA polymerase III (pol III). The study of the cis -acting elements and trans -acting factors involved in transcription of pRNA genes has revealed that the TATA-box-binding protein TBP, first thought to be involved only in transcription of mRNA genes by RNA pol II as part of the TFIID complex, also participates in transcription of RNA pol III genes. In addition, RNA pol II transcription complexes can differ in their ability to recognize a termination signal, and these different elongation properties are determined by promoter elements. INTRODUCTION In eukaryotes, transcription is carried out by three different classes of RNA polymerases. Although the large variety of promoter structures found within the class II transcription units always suggested a correspondingly large variety of RNA pol II initiation complexes, these complexes were nevertheless thought to consist of factors specialized for RNA pol II transcription and therefore very different from the factors constituting RNA pol I and RNA pol III initiation complexes. In addition, elongating RNA pol II transcription complexes were thought to be of uniform composition and thus to have similar elongation properties, regardless of which promoter they were derived from. During the past few years, however, several findings have altered these views. First, the genes encoding several RNA polymerase subunits have been...