MOLECULAR-GENETICS OF THE RNA-POLYMERASE-II GENERAL TRANSCRIPTIONAL MACHINERY

Transcription initiation by RNA polymerase II (RNA pol II) requires in teraction between cis-acting promoter elements and trans-acting factor s. The eukaryotic promoter consists of core elements, which include th e TATA box and other DNA sequences that define transcription start sit es, and regulatory elements, which either enhance or repress transcrip tion in a gene-specific manner. The core promoter is the site for asse mbly of the transcription preinitiation complex; which includes RNA po l II and the general transcription fctors TBP, TFIIB, TFIIE, TFIIF, an d TFIIH. Regulatory elements bind gene-specific factors, which affect the rate of transcription by interacting either directly or indirectly , with components of the general transcriptional machinery. A third cl ass of transcription factors, termed coactivators, is not required far basal transcription in vitro but often mediates activation by a broad spectrum of activators. Accordingly coactivators are neither gene-spe cific nor general transcription factors, although gene-specific coacti vators have been described in metazoan systems. Transcriptional repres sors include both gene-specific and general factors. Similar to coacti vators, general transcriptional repressors affect the expression of a broad spectrum of genes yet do not repress all genes. General represso rs either act through the core transcriptional machinery or are histon e related and presumably affect chromatin function. This review focuse s on the global effectors of RNA polymerase II transcription in yeast, including the general transcription factors, the coactivators, and th e general repressors. Emphasis is placed on the role that yeast geneti cs has played in identifying these factors and their associated functi ons.